MMUSIC J. Rosenberg
Internet-Draft Cisco
Intended status: Standards Track March 5, 2007
Expires: September 6, 2007
Interactive Connectivity Establishment (ICE): A Methodology for Network
Address Translator (NAT) Traversal for Offer/Answer Protocols
draft-ietf-mmusic-ice-14
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Copyright (C) The IETF Trust (2007).
Abstract
This document describes a protocol for Network Address Translator
(NAT) traversal for multimedia sessions established with the offer/
answer model. This protocol is called Interactive Connectivity
Establishment (ICE). ICE makes use of the Session Traversal
Utilities for NAT (STUN) protocol, applying its binding discovery and
relay usages, in addition to defining a new usage for checking
connectivity between peers. ICE can be used by any protocol
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Internet-Draft ICE March 20071. IntroductionRFC 3264 [4] defines a two-phase exchange of Session Description
Protocol (SDP) messages [10] for the purposes of establishment of
multimedia sessions. This offer/answer mechanism is used by
protocols such as the Session Initiation Protocol (SIP) [3].
Protocols using offer/answer are difficult to operate through Network
Address Translators (NAT). Because their purpose is to establish a
flow of media packets, they tend to carry the IP of media sources and
sinks within their messages, which is known to be problematic through
NAT [16]. The protocols also seek to create a media flow directly
between participants, so that there is no application layer
intermediary between them. This is done to reduce media latency,
decrease packet loss, and reduce the operational costs of deploying
the application. However, this is difficult to accomplish through
NAT. A full treatment of the reasons for this is beyond the scope of
this specification.
Numerous solutions have been proposed for allowing these protocols to
operate through NAT. These include Application Layer Gateways
(ALGs), the Middlebox Control Protocol [17], Simple Traversal
Underneath NAT (STUN) [15] and its revision, retitled Session
Traversal Utilities for NAT [12], the STUN Relay Usage [13], and
Realm Specific IP [19] [20] along with session description extensions
needed to make them work, such as the Session Description Protocol
(SDP) [10] attribute for the Real Time Control Protocol (RTCP) [2].
Unfortunately, these techniques all have pros and cons which make
each one optimal in some network topologies, but a poor choice in
others. The result is that administrators and implementors are
making assumptions about the topologies of the networks in which
their solutions will be deployed. This introduces complexity and
brittleness into the system. What is needed is a single solution
which is flexible enough to work well in all situations.
This specification defines Interactive Connectivity Establishment
(ICE) as a technique for NAT traversal for media streams established
by the offer/answer model. ICE is an extension to the offer/answer
model, and works by including a multiplicity of IP addresses and
ports in SDP offers and answers, which are then tested for
connectivity by peer-to-peer STUN exchanges. The IP addresses and
ports included in the SDP are gathered using the STUN binding
acquisition techniques in [12] and relay allocation procedures in
[13].
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Internet-Draft ICE March 20072. Overview of ICE
In a typical ICE deployment, we have two endpoints (known as AGENTS
in RFC 3264 terminology) which want to communicate. They are able to
communicate indirectly via some signaling protocol (such as SIP), by
which they can perform an offer/answer exchange of SDP [4] messages.
Note that ICE is not intended for NAT traversal for SIP, which is
assumed to be provided via another mechanism [35]. At the beginning
of the ICE process, the agents are ignorant of their own topologies.
In particular, they might or might not be behind a NAT (or multiple
tiers of NATs). ICE allows the agents to discover enough information
about their topologies to potentially find one or more paths by which
they can communicate.
Figure 1 shows a typical environment for ICE deployment. The two
endpoints are labelled L and R (for left and right, which helps
visualize call flows). Both L and R are behind their own respective
NATs though they may not be aware of it. The type of NAT and its
properties are also unknown. Agents L and R are capable of engaging
in an offer/answer exchange by which they can exchange SDP messages,
whose purpose is to set up a media session between L and R.
Typically, this exchange will occur through a SIP server.
In addition to the agents, a SIP server and NATs, ICE is typically
used in concert with STUN servers in the network. Each agent can
have its own STUN server, or they can be the same.
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until it finds one or more that works.
2.1. Gathering Candidate Addresses
In order to execute ICE, an agent has to identify all of its address
candidates. A CANDIDATE is a transport address - a combination of IP
address and port for a particular transport protocol. This document
defines three types of candidates, some derived from physical or
logical network interfaces, others discoverable via STUN. Naturally,
one viable candidate is a transport address obtained directly from a
local interface. Such a candidate is called a HOST CANDIDATE. The
local interface could be ethernet or WiFi, or it could be one that is
obtained through a tunnel mechanism, such as a Virtual Private
Network (VPN) or Mobile IP (MIP). In all cases, such a network
interface appears to the agent as a local interface from which ports
(and thus a candidate) can be allocated.
If an agent is multihomed, it obtains a candidate from each
interface. Depending on the location of the PEER (the other agent in
the session) on the IP network relative to the agent, the agent may
be reachable by the peer through one or more of those interfaces.
Consider, for example, an agent which has a local interface to a
private net 10 network (I1), and a second connected to the public
Internet (I2). A candidate from I1 will be directly reachable when
communicating with a peer on the same private net 10 network, while a
candidate from I2 will be directly reachable when communicating with
a peer on the public Internet. Rather than trying to guess which
interface will work prior to sending an offer, the offering agent
includes both candidates in its offer.
Next, the agent uses STUN to obtain additional candidates. These
come in two flavors: translated addresses on the public side of a NAT
(SERVER REFLEXIVE CANDIDATES) and addresses of media relays (RELAYED
CANDIDATES). The relationship of these candidates to the host
candidate is shown in Figure 2. Both types of candidates are
discovered using STUN.
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To Internet
|
|
| /------------ Relayed
Y:y | / Address
+--------+
| |
| STUN |
| Server |
| |
+--------+
|
|
| /------------ Server
X1':x1'|/ Reflexive
+------------+ Address
| NAT |
+------------+
|
| /------------ Local
X:x |/ Address
+--------+
| |
| Agent |
| |
+--------+
Figure 2: Candidate Relationships
To find a server reflexive candidate, the agent sends a STUN Binding
Request, using the Binding Discovery Usage [12] from each host
candidate, to its STUN server. It is assumed that the address of the
STUN server is manually configured or learned in some unspecified
way. It is RECOMMENDED that when an agent has a choice of STUN
servers (when, for example, they are learned through DNS records and
multiple results are returned), an agent uses a single STUN server
(based on its IP address) for all candidates for a particular
session. This improves the performance of ICE.
When the agent sends the Binding Request from IP address and port
X:x, the NAT (assuming there is one) will allocate a binding X1':x1',
mapping this server reflexive candidate to the host candidate X:x.
Outgoing packets sent from the host candidate will be translated by
the NAT to the server reflexive candidate. Incoming packets sent to
the server relexive candidate will be translated by the NAT to the
host candidate and forwarded to the agent. We call the host
candidate associated with a given server reflexive candidate the
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BASE.
NOTE: "Base" refers to the address an agent sends from for a
particular candidate. Thus, as a degenerate case host candidates
also have a base, but it's the same as the host candidate.
When there are multiple NATs between the agent and the STUN server,
the STUN request will create a binding on each NAT, but only the
outermost server reflexive candidate will be discovered by the agent.
If the agent is not behind a NAT, then the base candidate will be the
same as the server reflexive candidate and the server reflexive
candidate is redundant and will be eliminated.
The final type of candidate is a RELAYED CANDIDATE. The STUN Relay
Usage [13] allows a STUN server to act as a media relay, forwarding
traffic between L and R. In order to send traffic to L, R sends
traffic to the media relay at Y:y, and the relay forwards that to
X1':x1', which passes through the NAT where it is mapped to X:x and
delivered to L.
2.2. Connectivity Checks
Once L has gathered all of its candidates, it orders them in highest
to lowest priority and sends them to R over the signalling channel.
The candidates are carried in attributes in the SDP offer. When R
receives the offer, it performs the same gathering process and
responds with its own list of candidates. At the end of this
process, each agent has a complete list of both its candidates and
its peer's candidates. It pairs them up, resulting in CANDIDATE
PAIRS. To see which pairs work, the agent schedules a series of
CHECKS. Each check is a STUN transaction that the client will
perform on a particular candidate pair by sending a STUN request from
the local candidate to the remote candidate.
The basic principle of the connectivity checks is simple:
1. Sort the candidate pairs in priority order.
2. Send checks on each candidate pair in priority order.
3. Acknowledge checks received from the other agent.
With both agents performing a check on a candidate pair, the result
is a 4-way handshake:
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L R
- -
STUN request -> \ L's
<- STUN response / check
<- STUN request \ R's
STUN response -> / check
Figure 3: Basic Connectivity Check
It is important to note that the STUN requests are sent to and from
the exact same IP addresses and ports that will be used for media
(e.g., RTP and RTCP). Consequently, agents demultiplex STUN and RTP/
RTCP using contents of the packets, rather than the port on which
they are received. Fortunately, this demultiplexing is easy to do,
especially for RTP and RTCP.
Because STUN is used for the connectivity check, the STUN response
will contain the agent's translated transport address on the public
side any NATs between the agent and its peer. If this transport
address is different from other candidates the agent already learned,
it represents a new candidate, called a PEER REFLEXIVE CANDIDATE,
which then gets tested by ICE just the same as any other candidate.
As an optimization, as soon as R gets L's check message R immediately
sends a check message to L on the same candidate pair. This
accelerates the process of finding a valid candidate, and is called a
TRIGGERED CHECK.
At the end of this handshake, both L and R know that they can send
(and receive) messages end-to-end in both directions.
2.3. Sorting Candidates
Because the algorithm above searches all candidate pairs, if a
working pair exists it will eventually find it no matter what order
the candidates are tried in. In order to produce faster (and better)
results, the candidates are sorted in a specified order. The
resulting list of sorted candidate pairs is called the CHECK LIST.
The algorithm is described in Section 4.1.2 but follows two general
principles:
o Each agent gives its candidates a numeric priority which is sent
along with the candidate to the peer
o The local and remote priorities are combined so that each agent
has the same ordering for the candidate pairs.
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The second property is important for getting ICE to work when there
are NATs in front of L and R. Frequently, NATs will not allow packets
in from a host until the agent behind the NAT has sent a packet
towards that host. Consequently, ICE checks in each direction will
not succeed until both sides have sent a check through their
respective NATs.
The agent works through this check list by sending a STUN request for
the next candidate pair on the list every 20ms. These are called
PERIODIC CHECKS.
In general the priority algorithm is designed so that candidates of
similar type get similar priorities and so that more direct routes
(that is, through fewer media relays and through fewer NATs) are
preferred over indirect ones (ones with more media relays and more
NATs). Within those guidelines, however, agents have a fair amount
of discretion about how to tune their algorithms.
2.4. Frozen Candidates
The previous description only addresses the case where the agents
wish to establish a media session with one COMPONENT (a piece of a
media stream requiring a single transport address; a media stream may
require multiple components, each of which has to work for the media
stream as a whole to be work). Typically, (e.g., with RTP and RTCP)
the agents actually need to establish connectivity for more than one
flow.
The network properties are likely to be very similar for each
component (especially because RTP and RTCP are sent and received from
the same IP address). It is usually possible to leverage information
from one media component in order to determine the best candidates
for another. ICE does this with a mechanism called "frozen
candidates."
Each candidate is associated with a property called its FOUNDATION.
Two candidates have the same foundation when they are "similar" - of
the same type and obtained from the same interfaces and STUN servers.
Otherwise, their foundation is different. A candidate pair has a
foundation too, which is just the concatenation of the foundations of
its two candidates. Initially, only the candidate pairs with unique
foundations are tested. The other candidate pairs are marked
"frozen". When the connectivity checks for a candidate pair succeed,
the candidate pairs with the same foundation are unfrozen. This
avoids repeated checking of components which are superficially more
attractive but in fact are likely to fail.
While we've described "frozen" here as a separate mechanism for
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expository purposes, in fact it is an integral part of ICE and the
the ICE prioritization algorithm automatically ensures that the right
candidates are unfrozen and checked in the right order.
2.5. Security for Checks
Because ICE is used to discover which addresses can be used to send
media between two agents, it is important to ensure that the process
cannot be hijacked to send media to the wrong location. Each STUN
connectivity check is covered by a message authentication code (MAC)
computed using a key exchanged in the signalling channel. This MAC
provides message integrity and data origin authentication, thus
stopping an attacker from forging or modifying connectivity check
messages. The MAC also aids in disambiguating ICE exchanges from
forked calls when ICE is used with SIP [3].
2.6. Concluding ICE
ICE checks are performed in a specific sequence, so that high
priority candidate pairs are checked first, followed by lower
priority ones. One way to conclude ICE is to declare victory as soon
as a check for each component of each media stream completes
successfully. Indeed, this is a reasonable algorithm, and details
for it are provided below. However, it is possible that packet
losses will cause a higher priority check to take longer to complete.
In that case, allowing ICE to run a little longer might produce
better results. More fundamentally, however, the prioritization
defined by this specification may not yield "optimal" results. As an
example, if the aim is to select low latency media paths, usage of a
relay is a hint that latencies may be higher, but it is nothing more
than a hint. An actual RTT measurement could be made, and it might
demonstrate that a pair with lower priority is actually better than
one with higher priority.
Consequently, ICE assigns one of the agents in the role of the
CONTROLLING AGENT, and the other of the CONTROLLED AGENT. The
controlled agent gets to nominate which candidate pairs will get used
for media amongst the ones that are valid. It can do this in one of
two ways - using REGULAR NOMINATION or AGGRESSIVE NOMINATION.
With regular nomination, the controlling agent lets the checks
continue until at least one valid candidate pair for each media
stream is found. Then, it picks amongst those that are valid, and
sends a second STUN request on its NOMINATED candidate pair, but this
time with a flag set to tell the peer that this pair has been
nominated for use. A This is shown in Figure 4.
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L R
- -
STUN request \ L's
<- STUN response / check
<- STUN request \ R's
STUN response -> / check
STUN request + flag \ L's
<- STUN response / check
Figure 4: Regular Nomination
Once the STUN transaction with the flag completes, both sides cancel
any future checks for that media stream. ICE will now send media
using this pair. The pair an ICE agent is using for media is called
the SELECTED PAIR.
In aggressive nomination, the controlling agent puts the flag in
every STUN request it sends. This way, once the first check
succeeds, ICE processing is complete for that media stream and the
controlling agent doesn't have to send a second STUN request. The
selected pair will be the highest priority valid pair. Aggressive
nomination is faster than regular nomination, but gives less
flexibility. Aggressive nomination is shown in Figure 5.
L R
- -
STUN request + flag \ L's
<- STUN response / check
<- STUN request \ R's
STUN response -> / check
Figure 5: Aggressive Nomination
Once all of the media streams are completed, the controlling endpoint
sends an updated offer if the candidates in the m and c lines for the
media stream (called the DEFAULT CANDIDATES) don't match ICE's
SELECTED CANDIDATES.
Once ICE is concluded, it can be restarted at any time for one or all
of the media streams by either agent. This is done by sending an
updated offer indicating a restart.
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Internet-Draft ICE March 20072.7. Lite Implementations
In order for ICE to be used in a call, both agents need to support
it. However, certain agents will always be connected to the public
Internet and have a public IP address at which it can receive packets
from any correspondent. To make it easier for these devices to
support ICE, ICE defines a special type of implementation called LITE
(in contrast to the normal FULL implementation). A lite
implementation doesn't gather candidates; it includes only host
candidates for any media stream. When a lite implementation connects
with a full implementation, the full agent takes the role of the
controlling agent, and the lite agent takes on the controlled role.
In addition, lite agents do not need to generate connectivity checks,
run the state machines, or compute candidate pairs. Additional
guidance on when a lite implementation is appropriate, see the
discussion in Appendix A. For an informational summary of ICE
processing as seen by a lite agent, see [36].
It is important to note that the lite implementation was added to
this specification to provide a stepping stone to full
implementation. Even for devices that are always connected to the
public Internet, a full implementation is preferable if achievable.
3. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].
Readers should be familiar with the terminology defined in the offer/
answer model [4], STUN [12] and NAT Behavioral requirements for UDP
[29]
This specification makes use of the following additional terminology:
Agent: As defined in RFC 3264, an agent is the protocol
implementation involved in the offer/answer exchange. There are
two agents involved in an offer/answer exchange.
Peer: From the perspective of one of the agents in a session, its
peer is the other agent. Specifically, from the perspective of
the offerer, the peer is the answerer. From the perspective of
the answerer, the peer is the offerer.
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Transport Address: The combination of an IP address and transport
protocol (such as UDP or TCP) port.
Candidate: A transport address that is to be tested by ICE
procedures in order to determine its suitability for usage for
receipt of media. Candidates also have properties - their type
(server reflexive, relayed or host), priority, foundation, and
base.
Component: A component is a piece of a media stream requiring a
single transport address; a media stream may require multiple
components, each of which has to work for the media stream as a
whole to work. For media streams based on RTP, there are two
components per media stream - one for RTP, and one for RTCP.
Host Candidate: A candidate obtained by binding to a specific port
from an interface on the host. This includes both physical
interfaces and logical ones, such as ones obtained through Virtual
Private Networks (VPNs) and Realm Specific IP (RSIP) [19] (which
lives at the operating system level).
Server Reflexive Candidate: A candidate obtained by sending a STUN
request from a host candidate to a STUN server, distinct from the
peer. The STUN server's address is configured or learned by the
client prior to an offer/answer exchange.
Peer Reflexive Candidate: A candidate obtained by sending a STUN
request from a host candidate to the STUN server running on a
peer's candidate.
Relayed Candidate: A candidate obtained by sending a STUN Allocate
request from a host candidate to a STUN server. The relayed
candidate is resident on the STUN server, and the STUN server
relays packets back towards the agent.
Base: The base of a server reflexive candidate is the host candidate
from which it was derived. A host candidate is also said to have
a base, equal to that candidate itself. Similarly, the base of a
relayed candidate is that candidate itself.
Foundation: An arbitrary string that is the same for two candidates
that have the same type, base IP address, and STUN server. If any
of these are different then the foundation will be different. Two
candidate pairs with the same foundation pairs are likely to have
similar network characteristics. Foundations are used in the
frozen algorithm.
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Local Candidate: A candidate that an agent has obtained and included
in an offer or answer it sent.
Remote Candidate: A candidate that an agent received in an offer or
answer from its peer.
Default Destination/Candidate: The default destination for a
component of a media stream is the transport address that would be
used by an agent that is not ICE aware. For the RTP component,
the default IP address is in the c line of the SDP, and the port
in the m line. For the RTCP component it is in the rtcp attribute
when present, and when not present, the IP address in the c line
and 1 plus the port in the m line. A default candidate for a
component is one whose transport address matches the default
destination for that component.
Candidate Pair: A pairing containing a local candidate and a remote
candidate.
Check, Connectivity Check, STUN Check: A STUN Binding Request
transaction for the purposes of verifying connectivity. A check
is sent from the local candidate to the remote candidate of a
candidate pair.
Check List: An ordered set of candidate pairs that an agent will use
to generate checks.
Periodic Check: A connectivity check generated by an agent as a
consequence of a timer that fires periodically, instructing it to
send a check.
Triggered Check: A connectivity check generated as a consequence of
the receipt of a connectivity check from the peer.
Valid List: An ordered set of candidate pairs for a media stream
that have been validated by a successful STUN transaction.
Full: An ICE implementation that performs the complete set of
functionality defined by this specification.
Lite: An ICE implementation that omits certain functions,
implementing only as much as is necessary for a peer
implementation that is full to gain the benefits of ICE. Lite
implementations can only act as the controlled agent in a session,
and do not gather candidates.
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Controlling Agent: The STUN agent which is responsible for selecting
the final choice of candidate pairs and signaling them through
STUN and an updated offer, if needed. In any session, one agent
is always controlling. The other is the controlled agent.
Controlled Agent: A STUN agent which waits for the controlling agent
to select the final choice of candidate pairs.
Regular Nomination: The process of picking a valid candidate pair
for media traffic by validating the pair with one STUN request,
and then picking it by sending a second STUN request with a flag
indicating its nomination.
Aggressive Nomination: The process of picking a valid candidate pair
for media traffic by including a flag in every STUN request, such
that the first one to produce a valid candidate pair is used for
media.
Nominated: If a valid candidate pair has its nominated flag set, it
means that it may be selected by ICE for sending and receiving
media.
Selected Pair, Selected Candidate: The candidate pair selected by
ICE for sending and receiving media is called the selected pair,
and each of its candidates is called the selected candidate.
4. Sending the Initial Offer
In order to send the initial offer in an offer/answer exchange, an
agent must (1) gather candidates, (2) prioritize them, (3) choose
default candidates, and then (4) formulate and send the SDP. The
first of these four steps differ for full and lite implementations.
4.1. Full Implementation Requirements4.1.1. Gathering Candidates
An agent gathers candidates when it believes that communications is
imminent. An offerer can do this based on a user interface cue, or
based on an explicit request to initiate a session. Every candidate
is a transport address. It also has a type and a base. Three types
are defined and gathered by this specification - host candidates,
server reflexive candidates, and relayed candidates. The server
reflexive and relayed candidates are gathered using STUN's Binding
Discovery and Relay Usages. The base of a candidate is the candidate
that an agent must send from when using that candidate.
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The first step is to gather host candidates. Host candidates are
obtained by binding to ports (typically ephemeral) on an interface
(physical or virtual, including VPN interfaces) on the host. The
process for gathering host candidates depends on the transport
protocol. Procedures are specified here for UDP.
For each UDP media stream the agent wishes to use, the agent SHOULD
obtain a candidate for each component of the media stream on each
interface that the host has. It obtains each candidate by binding to
a UDP port on the specific interface. A host candidate (and indeed
every candidate) is always associated with a specific component for
which it is a candidate. Each component has an ID assigned to it,
called the component ID. For RTP-based media streams, the RTP itself
has a component ID of 1, and RTCP a component ID of 2. If an agent
is using RTCP it MUST obtain a candidate for it. If an agent is
using both RTP and RTCP, it would end up with 2*K host candidates if
an agent has K interfaces.
The base for each host candidate is set to the candidate itself.
4.1.1.2. Server Reflexive and Relayed Candidates
Agents SHOULD obtain relayed candidates and MUST obtain server
reflexive candidates. The requirement to obtain relayed candidates
is at SHOULD strength to allow for provider variation. Use of relays
is expensive, and when ICE is being used, relays will only be
required when both endpoints are behind NATs that perform address and
port dependent mapping. Consequently, some deployments might
consider this use case to be marginal, and elect not to use relays.
If they are not used, it is RECOMMENDED that it be implemented and
just disabled through configuration, so that it can re-enabled
through configuration if conditions change in the future.
The agent next pairs each host candidate with the STUN server with
which it is configured or has discovered by some means. This
specification only considers usage of a single STUN server. At that
very instance, and then every Ta milliseconds thereafter, the agent
chooses another such pair (the order is inconsequential), and sends a
STUN request to the server from that host candidate. If the agent is
using both relayed and server reflexive candidates, this request MUST
be a STUN Allocate request using the relay usage [13]. If the agent
is using only server reflexive candidates, the request MUST be a STUN
Binding request using the binding discovery usage [12].
The value of Ta SHOULD be configurable, and SHOULD have a default of
20ms (see Appendix B.1 for a discussion on the selection of this
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value). Note that this pacing applies only to starting STUN
transactions with source and destination transport addresses (i.e.,
the host candidate and STUN server respectively) for which a STUN
transaction has not previously been sent. Consequently,
retransmissions of a STUN request are governed entirely by the
retransmission rules defined in [12]. Similarly, retries of a
request due to recoverable errors (such as an authentication
challenge) happen immediately and are not paced by timer Ta. Because
of this pacing, it will take a certain amount of time to obtain all
of the server reflexive and relayed candidates. Implementations
should be aware of the time required to do this, and if the
application requires a time budget, limit the number of candidates
which are gathered.
The agent will receive a STUN Binding or Allocate response. A
successful Allocate Response will provide the agent with a server
reflexive candidate (obtained from the mapped address) and a relayed
candidate in the RELAY-ADDRESS attribute. If the Allocate request is
rejected because the server lacks resources to fulfill it, the agent
SHOULD instead send a Binding Request to obtain a server reflexive
candidate. A Binding Response will provide the agent with only a
server reflexive candidate (also obtained from the mapped address).
The base of the server reflexive candidate is the host candidate from
which the Allocate or Binding request was sent. The base of a
relayed candidate is that candidate itself. If a relayed candidate
is identical to a host candidate (which can happen in rare cases),
the relayed candidate MUST be discarded. Proper operation of ICE
depends on each base being unique.
4.1.1.3. Eliminating Redundant Candidates
Next, the agent eliminates redundant candidates. A candidate is
redundant if its transport address equals another candidate, and its
base equals the base of that other candidate. Note that two
candidates can have the same transport address yet have different
bases, and these would not be considered redundant.
4.1.1.4. Computing Foundations
Finally, the agent assigns each candidate a foundation. The
foundation is an identifier, scoped within a session. Two candidates
MUST have the same foundation ID when all of the following are true:
o they are of the same type (host, relayed, server reflexive, peer
reflexive or relayed)
o their bases have the same IP address (the ports can be different)
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o for reflexive and relayed candidates, the STUN servers used to
obtain them have the same IP address.
Similarly, two candidates MUST have different foundations if their
types are different, their bases have different IP addresses, or the
STUN servers used to obtain them have different IP addresses.
4.1.1.5. Keeping Candidates Alive
Once server reflexive and relayed candidates are allocated, they MUST
be kept alive until ICE processing has completed. For server
reflexive candidates learned through the Binding Discovery usage,
this MUST be another Binding Request from the Binding Discovery
usage. For relayed candidates learned through the Relay Usage, this
MUST be a new Allocate request. The Allocate request will also
refresh the server reflexive candidate.
4.1.2. Prioritizing Candidates
The prioritization process results in the assignment of a priority to
each candidate. Each candidate for a media stream MUST have a unique
priority that MUST be a positive integer between 1 and (2**32 - 1).
This priority will be used by ICE to determine the order of the
connectivity checks and the relative preference for candidates.
An agent SHOULD compute this priority using the formula in
Section 4.1.2.1 and choose its parameters using the guidelines in
Section 4.1.2.2. If an agent elects to use a different formula, ICE
will take longer to converge since both agents will not be
coordinated in their checks.
4.1.2.1. Recommended Formula
When using the formula, an agent computes the priority by determining
a preference for each type of candidate (server reflexive, peer
reflexive, relayed and host), and, when the agent is multihomed,
choosing a preference for its interfaces. These two preferences are
then combined to compute the priority for a candidate. That priority
is computed using the following formula:
priority = (2^24)*(type preference) +
(2^8)*(local preference) +
(2^0)*(256 - component ID)
The type preference MUST be an integer from 0 to 126 inclusive, and
represents the preference for the type of the candidate (where the
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types are local, server reflexive, peer reflexive and relayed). A
126 is the highest preference, and a 0 is the lowest. Setting the
value to a 0 means that candidates of this type will only be used as
a last resort. The type preference MUST be identical for all
candidates of the same type and MUST be different for candidates of
different types. The type preference for peer reflexive candidates
MUST be higher than that of server reflexive candidates. Note that
candidates gathered based on the procedures of Section 4.1.1 will
never be peer reflexive candidates; candidates of these type are
learned from the STUN connectivity checks performed by ICE.
The local preference MUST be an integer from 0 to 65535 inclusive.
It represents a preference for the particular interface from which
the candidate was obtained, in cases where an agent is multihomed.
65535 represents the highest preference, and a zero, the lowest.
When there is only a single interface, this value SHOULD be set to
65535. More generally, if there are multiple candidates for a
particular component for a particular media stream which have the
same type, the local preference MUST be unique for each one. In this
specification, this only happens for multi-homed hosts.
The component ID is the component ID for the candidate, and MUST be
between 1 and 256 inclusive.
4.1.2.2. Guidelines for Choosing Type and Local Preferences
One criteria for selection of the type and local preference values is
the use of an intermediary, such as a media relay. With an
intermediary, if media is sent to that candidate, it will first
transit the intermediary before being received. Relayed candidates
are one type of candidate that involves an intermediary. Another are
host candidates obtained from a VPN interface. When media is
transited through an intermediary, it can increase the latency
between transmission and reception. It can increase the packet
losses, because of the additional router hops that may be taken. It
may increase the cost of providing service, since media will be
routed in and right back out of a media relay run by the provider.
If these concerns are important, the type preference for relayed
candidates SHOULD be lower than host candidates. The RECOMMENDED
values are 126 for host candidates, 100 for server reflexive
candidates, 110 for peer reflexive candidates, and 0 for relayed
candidates. Furthermore, if an agent is multi-homed and has multiple
interfaces, the local preference for host candidates from a VPN
interface SHOULD have a priority of 0.
Another criteria for selection of preferences is IP address family.
ICE works with both IPv4 and IPv6. It therefore provides a
transition mechanism that allows dual-stack hosts to prefer
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connectivity over IPv6, but to fall back to IPv4 in case the v6
networks are disconnected (due, for example, to a failure in a 6to4
relay) [24]. It can also help with hosts that have both a native
IPv6 address and a 6to4 address. In such a case, higher local
preferences could be assigned to the v6 interface, followed by the
6to4 interfaces, followed by the v4 interfaces. This allows a site
to obtain and begin using native v6 addresses immediately, yet still
fallback to 6to4 addresses when communicating with agents in other
sites that do not yet have native v6 connectivity.
Another criteria for selecting preferences is security. If a user is
a telecommuter, and therefore connected to their corporate network
and a local home network, they may prefer their voice traffic to be
routed over the VPN in order to keep it on the corporate network when
communicating within the enterprise, but use the local network when
communicating with users outside of the enterprise. In such a case,
a VPN interface would have a higher local preference than any other
interface.
Another criteria for selecting preferences is topological awareness.
This is most useful for candidates that make use of relays. In those
cases, if an agent has preconfigured or dynamically discovered
knowledge of the topological proximity of the relays to itself, it
can use that to assign higher local preferences to candidates
obtained from closer relays.
4.1.3. Choosing Default Candidates
A candidate is said to be default if it would be the target of media
from a non-ICE peer; that target being called the DEFAULT
DESTINATION. If the default candidates are not selected by the ICE
algorithm when communicating with an ICE-aware peer, an updated
offer/answer will be required after ICE processing completes in order
to "correct" the SDP so that the default destination for media
matches the candidates selected by ICE. If ICE happens to select the
default candidates, no updated offer/answer is required.
An agent MUST choose a set of candidates, one for each component of
each in-use media stream, to be default. A media stream is in-use if
it does not have a port of zero (which is used in RFC 3264 to reject
a media stream). Consequently, a media stream is in-use even if it
is marked as a=inactive [10] or has a bandwidth value of zero.
It is RECOMMENDED that default candidates be chosen based on the
likelihood of those candidates to work with the peer that is being
contacted. It is RECOMMENDED that the default candidates are the
relayed candidates (if relayed candidates are available), server
reflexive candidates (if server reflexive candidates are available),
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and finally host candidates.
4.2. Lite Implementation
For each media stream, the agent allocates a single candidate for
each component of the media stream from one of its interfaces. If an
agent has multiple interfaces, it MUST choose one for each component
of a particular media stream. With the lite implementation, ICE
cannot be used to dynamically choose amongst candidates. Each
component has an ID assigned to it, called the component ID. For
RTP-based media streams the RTP itself has a component ID of 1, and
RTCP a component ID of 2. If an agent is using RTCP it MUST obtain a
candidate for it.
Each candidate is assigned a foundation. The foundation MUST be
different for two candidates from different interfaces, and MUST be
the same otherwise. A simple integer that increments for each
interface will suffice. In addition, each candidate MUST be assigned
a unique priority amongst all candidates for the same media stream.
This priority SHOULD be equal to 2^24*(126) + 2^8*(65535) + 256 minus
the component ID, which is 2130706432 minus the component ID.
If an agent has included two candidates for a component, the v4
candidate SHOULD be selected as the default. Since a lite
implementation has a single candidate for a component, each of these
candidates is considered to be default.
4.3. Encoding the SDP
The process of encoding the SDP is identical between full and lite
implementations.
The agent will include an m-line for each media stream it wishes to
use. The ordering of media streams in the SDP is relevant for ICE.
ICE will perform its connectivity checks for the first m-line first,
and consequently media will be able to flow for that stream first.
Agents SHOULD place their most important media stream, if there is
one, first in the SDP.
There will be a candidate attribute for each candidate for a
particular media stream. Section 15 provides detailed rules for
constructing this attribute. The attribute carries the IP address,
port and transport protocol for the candidate, in addition to its
properties that need to be signaled to the peer for ICE to work: the
priority, foundation, and component ID. The candidate attribute also
carries information about the candidate that is useful for
diagnostics and other functions: its type and related transport
addresses.
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STUN connectivity checks between agents make use of a short term
credential that is exchanged in the offer/answer process. The
username part of this credential is formed by concatenating a
username fragment from each agent, separated by a colon. Each agent
also provides a password, used to compute the message integrity for
requests it receives. The username fragment and password are
exchanged in the ice-ufrag and ice-pwd attributes, respectively. In
addition to providing security, the username provides disambiguation
and correlation of checks to media streams. See Appendix B.4 for
motivation.
If an agent is a lite implementation, it MUST include an "a=ice-lite"
session level attribute in its SDP. If an agent is a full
implementation, it MUST NOT include this attribute.
The default candidates are added to the SDP as the default
destination for media. For streams based on RTP, this is done by
placing the IP address and port of the RTP candidate into the c and m
lines, respectively. If the agent is utilizing RTCP, it MUST encode
the RTCP candidate using the a=rtcp attribute as defined in RFC 3605
[2]. If RTCP is not in use, the agent MUST signal that using b=RS:0
and b=RR:0 as defined in RFC 3556 [5].
The transport addresses that will be the default destination for
media when communicating with non-ICE peers MUST also be present as
candidates in one or more a=candidate lines.
ICE provides for extensibility by allowing an offer or answer to
contain a series of tokens which identify the ICE extensions used by
that agent. If an agent supports an ICE extension, it MUST include
the token defined for that extension in the ice-options attribute.
The following is an example SDP message that includes ICE attributes
(lines folded for readability):
v=0
o=jdoe 2890844526 2890842807 IN IP4 10.0.1.1
s=
c=IN IP4 192.0.2.3
t=0 0
a=ice-pwd:asd88fgpdd777uzjYhagZg
a=ice-ufrag:8hhY
m=audio 45664 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=candidate:1 1 UDP 2130706178 10.0.1.1 8998 typ local
a=candidate:2 1 UDP 1694498562 192.0.2.3 45664 typ srflx raddr
10.0.1.1 rport 8998
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Once an agent has sent its offer or sent its answer, that agent MUST
be prepared to receive both STUN and media packets on each candidate.
As discussed in Section 11.1, media packets can be sent to a
candidate prior to its appearance as the default destination for
media in an offer or answer.
5. Receiving the Initial Offer
When an agent receives an initial offer, it will check if the offeror
supports ICE, determine its own role, gather candidates, prioritize
them, choose default candidates, encode and send an answer, and for
full implementations, form the check lists and begin connectivity
checks.
5.1. Verifying ICE Support
The answerer will proceed with the ICE procedures defined in this
specification if the following are all true:
o For each media stream, the default destination for at least one
component of the media stream appears in a candidate attribute.
For example, in the case of RTP, the IP address and port in the c
and m line, respectively, appears in a candidate attribute, or the
value in the rtcp attribute appears in a candidate attribute.
o The offer omitted an a=ice-lite attribute or the answerer is a
full implementation.
If any of these conditions are not met, the agent MUST process the
SDP based on normal RFC 3264 procedures, without using any of the ICE
mechanisms described in the remainder of this specification with the
following exceptions:
1. The agent MUST follow the rules of Section 10, which describe
keepalive procedures for all agents.
2. If the agent is not proceeding with ICE because there were
a=candidate attributes, but none that matched the default
destination of the media stream, the agent MUST include an a=ice-
mismatch attribute in its answer.
5.2. Determining Role
For each session, each agent takes on a role. There are two roles -
controlling, and controlled. The controlling agent is responsible
for nominating the candidate pairs that can be used by ICE for each
media stream, and for generating the updated offer based on ICE's
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selection, when needed. The controlled agent is told which candidate
pairs to use for each media stream, and does not generate an updated
offer to signal this information.
If one of the agents is a lite implementation, it MUST assume the
controlled role, and its peer (which will be full; if it was lite,
ICE would have aborted) MUST assume the controlling role. If the
agent and its peer are both full implementations, the agent which
generated the offer which started the ICE processing takes on the
controlling role, and the other takes the controlled role.
Based on this definition, once roles are determined for a session,
they persist unless ICE is restarted. A ICE restart (Section 9.1
causes a new selection of roles.
5.3. Gathering Candidates
The process for gathering candidates at the answerer is identical to
the process for the offerer as described in Section 4.1.1 for full
implementations and Section 4.2 for lite implementations. It is
RECOMMENDED that this process begin immediately on receipt of the
offer, prior to alerting the user. Such gathering MAY begin when an
agent starts.
5.4. Prioritizing Candidates
The process for prioritizing candidates at the answerer is identical
to the process followed by the offerer, as described in Section 4.1.2
for full implementations and Section 4.2 for lite implementations.
5.5. Choosing Default Candidates
The process for selecting default candidates at the answerer is
identical to the process followed by the offerer, as described in
Section 4.1.3 for full implementations and Section 4.2 for lite
implementations.
5.6. Encoding the SDP
The process for encoding the SDP at the answerer is identical to the
process followed by the offerer, as described in Section 4.3.
5.7. Forming the Check Lists
Forming check lists is done only by full implementations. Lite
implementations MUST skip the steps defined in this section.
There is one check list per in-use media stream resulting from the
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offer/answer exchange. To form the check list for a media stream,
the agent forms candidate pairs, computes a candidate pair priority,
orders the pairs by priority, prunes them, and sets their states.
These steps are described in this section.
5.7.1. Forming Candidate Pairs
First, the agent takes each of its candidates for a media stream
(called LOCAL CANDIDATES) and pairs them with the candidates it
received from its peer (called REMOTE CANDIDATES) for that media
stream. In order to prevent the attacks described in Section 17.4.2,
agents MAY limit the number of candidates they'll accept in an offer
or answer. A local candidate is paired with a remote candidate if
and only if the two candidates have the same component ID and have
the same IP address version. It is possible that some of the local
candidates don't get paired with a remote candidate, and some of the
remote candidates don't get paired with local candidates. This can
happen if one agent didn't include candidates for the all of the
components for a media stream. If this happens, the number of
components for that media stream is effectively reduced, and
considered to be equal to the minimum across both agents of the
maximum component ID provided by each agent across all components for
the media stream.
In the case of RTP, this would happen when one agent provided
candidates for RTCP, and the other did not. As another example, the
offerer can multiplex RTP and RTCP on the same port and signals it
can do that in the SDP through some new attribute. However, since
the offerer doesn't know if the answerer can perform such
multiplexing, the offerer includes candidates for RTP and RTCP on
separate ports, so that the offer has two components per media
stream. If the answerer can perform such multiplexing, it would
include just a single component for each candidate - for the combined
RTP/RTCP mux. ICE would end up acting as if there was just a single
component for this candidate.
The candidate pairs whose local and remote candidates were both the
default candidates for a particular component is called,
unsurprisingly, the default candidate pair for that component. This
is the pair that would be used to transmit media if both agents had
not been ICE aware.
In order to aid understanding, Figure 8 shows the relationships
between several key concepts - transport addresses, candidates,
candidate pairs, and check lists, in addition to indicating the main
properties of candidates and candidate pairs.
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Once the pairs are formed, a candidate pair priority is computed.
Let O be the priority for the candidate provided by the offerer. Let
A be the priority for the candidate provided by the answerer. The
priority for a pair is computed as:
pair priority = 2^32*MIN(O,A) + 2*MAX(O,A) + (O>A?1:0)
Where O-P>A-P?1:0 is an expression whose value is 1 if O-P is greater
than A-P, and 0 otherwise. This formula ensures a unique priority
for each pair in most cases. Once the priority is assigned, the
agent sorts the candidate pairs in decreasing order of priority. If
two pairs have identical priority, the ordering amongst them is
arbitrary.
5.7.3. Pruning the Pairs
This sorted list of candidate pairs is used to determine a sequence
of connectivity checks that will be performed. Each check involves
sending a request from a local candidate to a remote candidate.
Since an agent cannot send requests directly from a reflexive
candidate, but only from its base, the agent next goes through the
sorted list of candidate pairs. For each pair where the local
candidate is server reflexive, the server reflexive candidate MUST be
replaced by its base. Once this has been done, the agent MUST prune
the list. This is done by removing a pair if its local and remote
candidates are identical to the local and remote candidates of a pair
higher up on the priority list. The result is a sequence of ordered
candidate pairs, called the check list for that media stream.
In addition, in order to limit the attacks described in
Section 17.4.2, an agent SHOULD limit the total number of
connectivity checks they perform across all check lists to 100, by
discarding the lower priority candidate pairs until there are less
than 100.
5.7.4. Computing States
Each candidate pair in the check list has a foundation and a state.
The foundation is the combination of the foundations of the local and
remote candidates in the pair. The state is assigned once the check
list for each media stream has been computed. There are five
potential values that the state can have:
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Waiting: A check has not been performed for this pair, and can be
performed as soon as it is the highest priority Waiting pair on
the check list.
In-Progress: A check has been sent for this pair, but the
transaction is in progress.
Succeeded: A check for this pair was already done and produced a
successful result.
Failed: A check for this pair was already done and failed, either
never producing any response or producing an unrecoverable failure
response.
Frozen: A check for this pair hasn't been performed, and it can't
yet be performed until some other check succeeds, allowing this
pair to unfreeze and move into the Waiting state.
As ICE runs, the pairs will move between states as shown in Figure 9.
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2. It takes the first pair in the check list for the first media
stream (a media stream is the first media stream when it is
described by the first m-line in the SDP offer and answer), and
sets its state to Waiting.
3. It finds all of the other pairs in that check list with the same
component ID, but different foundations, and sets all of their
states to Waiting as well.
One of the check lists will have some number of pairs in the Waiting
state, and the other check lists will have all of their pairs in the
Frozen state. A check list with at least one pair that is not Frozen
is called an active check list.
The check list itself is associated with a state, which captures the
state of ICE checks for that media stream. There are two states:
Running: In this state, ICE checks are still in progress for this
media stream.
Completed: In this state, ICE checks have completed for this media
stream.
When a check list is first constructed as the consequence of an
offer/answer exchange, it is placed in the Running state.
ICE processing across all media streams also has a state associated
with it. This state is equal to Running while checks are in
progress. The state is Completed when all checks have been
completed. Rules for transitioning between states are described
below.
5.8. Performing Periodic Checks
Checks are generated only by full implementations. Lite
implementations MUST skip the steps described in this section.
An agent performs periodic checks and triggered checks. Periodic
checks occur periodically for each media stream, and involve choosing
the highest priority pair in the Waiting state from each check list,
and sending a check on it. Triggered checks are performed on receipt
of a connectivity check from the peer (see Section 7.2.1.3). This
section describes how periodic checks are performed.
Once the agent has computed the check lists as described in
Section 5.7, it sets a timer for each active check list. The timer
fires every Ta/N seconds, where N is the number of active check lists
(initially, there is only one active check list). Implementations
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MAY set the timer to fire less frequently than this. Ta is the same
value used to pace the gathering of candidates, as described in
Section 4.1.1. Dividing by N allows this aggregate check throughput
to be split between all active check lists. The first timer for each
active check list fires immediately, so that the agent performs a
connectivity check the moment the offer/answer exchange has been
done, followed by the next periodic check Ta seconds later.
When the timer fires, the agent MUST:
o Find the highest priority pair in that check list that is in the
Waiting state.
o If there is such a pair:
* Send a STUN check from the local candidate of that pair to the
remote candidate of that pair. The procedures for forming the
STUN request for this purpose are described in Section 7.1.1.
o If there is no such pair:
* Find the highest priority pair in that check list that is in
the Frozen state.
* If there is such a pair:
+ Unfreeze the pair.
+ Perform a check for that pair, causing its state to
transition to In-Progress.
* If there is no such pair:
+ Set the state of the check list to Completed.
+ Terminate the timer for that check list.
To compute the message integrity for the check, the agent uses the
remote username fragment and password learned from the SDP from its
peer. The local username fragment is known directly by the agent for
its own candidate.
6. Receipt of the Initial Answer
This section describes the procedures that an agent follows when it
receives the answer from the peer. It verifies that its peer
supports ICE, determines its role, and for full implementations,
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forms the check list and begins performing periodic checks.
6.1. Verifying ICE Support
The offerer will proceed with the ICE procedures defined in this
specification if there is at least one a=candidate attribute for each
media stream in the answer it just received. If this condition is
not met, the agent MUST process the SDP based on normal RFC 3264
procedures, without using any of the ICE mechanisms described in the
remainder of this specification, with the exception of Section 10,
which describes keepalive procedures.
In some cases, the answer may omit a=candidate attributes for the
media streams, and instead include an a=ice-mismatch attribute for
one or more of the media streams in the SDP. This signals to the
offerer that the answerer supports ICE, but that ICE processing was
not used for the session because an intermediary modified the default
destination for media components without modifying the corresponding
candidate attributes. See Section 17 for a discussion of cases where
this can happen. This specification provides no guidance on how an
agent should proceed in such a failure case.
6.2. Determining Role
The offerer follows the same procedures described for the answerer in
Section 5.2.
6.3. Forming the Check List
Formation of check lists is performed only by full implementations.
The offerer follows the same procedures described for the answerer in
Section 5.7.
6.4. Performing Periodic Checks
Periodic checks are performed only by full implementations. The
offerer follows the same procedures described for the answerer in
Section 5.8.
7. Performing Connectivity Checks
This section describes how connectivity checks are performed. All
ICE implementations are required to be compliant to [12], as opposed
to the older [15]. However, whereas a full implementation will both
generate checks (acting as a STUN client) and receive them (acting as
a STUN server), a lite implementation will only ever receive checks,
and thus will only act as a STUN server.
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These procedures define how an agent sends a connectivity check,
whether it is a periodic or a triggered check. These procedures are
only applicable to full implementations.
7.1.1. Sending the Request
The check is generated by sending a Binding Request from a local
candidate, to a remote candidate. [12] describes how Binding Requests
are constructed and generated. This section defines additional
procedures involving the PRIORITY and USE-CANDIDATE attributes,
defined for the connectivity check usage, and details how credentials
for message integrity and diffserv markings are computed.
7.1.1.1. PRIORITY and USE-CANDIDATE
An agent MUST include the PRIORITY attribute in its Binding Request.
The attribute MUST be set equal to the priority that would be
assigned, based on the algorithm in Section 4.1.2, to a peer
reflexive candidate, should one be learned as a consequence of this
check (see Section 7.1.2.2.1 for how peer reflexive candidates are
learned). This priority value will be computed identically to how
the priority for the local candidate of the pair was computed, except
that the type preference is set to the value for peer derived
candidate types.
The controlling agent MAY include the USE-CANDIDATE attribute in the
Binding Request. The controlled agent MUST NOT include it in its
Binding Request. This attribute signals that the controlling agent
wishes to cease checks for this component, and use the candidate pair
resulting from the check for this component. Section 8.1 provides
guidance on determining when to include it.
7.1.1.2. Forming Credentials
A Binding Request serving as a connectivity check MUST utilize a STUN
short term credential. The agent MUST include the USERNAME and
MESSAGE-INTEGRITY attributes. An agent MUST NOT wait to be
challenged for short term credentials. Rather, it MUST provide them
in each Binding Request.
Rather than being learned from a Shared Secret request, the short
term credential is exchanged in the offer/answer procedures. In
particular, the username is formed by concatenating the username
fragment provided by the peer with the username fragment of the agent
sending the request, separated by a colon (":"). The password is
equal to the password provided by the peer. For example, consider
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the case where agent L is the offerer, and agent R is the answerer.
Agent L included a username fragment of LFRAG for its candidates, and
a password of LPASS. Agent R provided a username fragment of RFRAG
and a password of RPASS. A connectivity check from L to R (and its
response of course) utilize the username RFRAG:LFRAG and a password
of RPASS. A connectivity check from R to L (and its response)
utilize the username LFRAG:RFRAG and a password of LPASS.
7.1.1.3. DiffServ Treatment
If the agent is using Diffserv Codepoint markings [27] in its media
packets, it SHOULD apply those same markings to its connectivity
checks.
7.1.2. Processing the Response
When a Binding Response is received, it is correlated to its Binding
Request using the transaction ID, as defined in [12], which then ties
it to the candidate pair for which the Binding Request was sent.
7.1.2.1. Failure Cases
If the STUN transaction generates an ICMP error, or generates a STUN
error response that is unrecoverable (as defined in [12], or times
out, the agent sets the state of the pair to Failed.
The agent MUST check that the source IP address and port of the
response equals the destination IP address and port that the Binding
Request was sent to, and that the destination IP address and port of
the response match the source IP address and port that the Binding
Request was sent from. In other words, the source and destination
transport addresses in the request and responses are the symmetric.
If they are not symmetric, the agent sets the state of the pair to
Failed.
7.1.2.2. Success Cases
If the STUN transaction generated a response between 200 and 299, and
the source IP address and port of the response equals the destination
IP address and port that the Binding Request was sent to, and the
destination IP address and port of the response match the source IP
address and port that the Binding Request was sent from, the check
was a success.
7.1.2.2.1. Discovering Peer Reflexive Candidates
The agent checks the mapped address from the STUN response. If the
transport address does not match any of the local candidates that the
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agent knows about, the mapped address represents a new candidate - a
peer reflexive candidate. Like other candidates, it has a type,
base, priority and foundation. They are computed as follows:
o Its type is equal to peer reflexive.
o Its base is set equal to the local candidate of the candidate pair
from which the STUN check was sent.
o Its priority is set equal to the value of the PRIORITY attribute
in the Binding Request.
o Its foundation is selected as described in Section 4.1.1.
This peer reflexive candidate is then added to the list of local
candidates for the media stream. Its username fragment and password
are the same as all other local candidates for that media stream.
However, the peer reflexive candidate is not paired with other remote
candidates. This is not necessary; a valid pair will be generated
from it momentarily based on the procedures in Section 7.1.2.2.3. If
an agent wishes to pair the peer reflexive candidate with other
remote candidates besides the one in the valid pair that will be
generated, the agent MAY generate an updated offer which includes the
peer reflexive candidate. This will cause it to be paired with all
other remote candidates.
7.1.2.2.2. Updating Pair States
The agent sets the state of the pair that generated the check to
succeeded. The agent sees if the success for this pair can cause
other pairs to be unfrozen. There are three cases:
o If the pair had a component ID of 1, the agent MUST change the
states for all other Frozen pairs for the same media stream and
same foundation, but different component IDs, to Waiting.
o If the pair had a component ID equal to the number of components
for the media stream (where this is the actual number of
components being used, in cases where the number of components
signaled in the SDP differs from offerer to answerer), the agent
MUST change the state for all other Frozen pairs for the first
component of different media streams (and thus in different check
lists) but the same foundation, to Waiting.
o If the pair has any other component ID, no other pairs can be
unfrozen.
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Internet-Draft ICE March 20077.1.2.2.3. Constructing a Valid Pair
Next, the agent constructs a candidate pair whose local candidate
equals the mapped address of the response, and whose remote candidate
equals the destination address to which the request was sent. This
is called a valid pair, since it has been validated by a STUN
connectivity check. The valid pair may equal the pair that generated
the check, may equal a different pair in the check list, or may be a
pair not currently on any check list. If the pair equals the pair
that generated the check or is on a check list currently, it is also
added to the VALID LIST, which is maintained by the agent for each
media stream. This list is empty at the start of ICE processing, and
fills as checks are performed, resulting in valid candidate pairs.
It will be very common that the pair will not be on any check list.
Recall that the check list has pairs whose local candidates are never
server reflexive; those pairs had their local candidates converted to
the base of the server reflexive candidates, and then pruned if they
were redundant. When the response to the STUN check arrives, the
mapped address will be reflexive if there is a NAT between the two.
In that case, the valid pair will have a local candidate that doesn't
match any of the pairs in the check list.
If the pair is not on any check list, the agent computes the priority
for the pair based on the priority of each candidate, using the
algorithm in Section 5.7. The priority of the local candidate
depends on its type. If it is not peer reflexive, it is equal to the
priority signaled for that candidate in the SDP. If it is peer
reflexive, it is equal to the PRIORITY attribute the agent placed in
the Binding Request which just completed. The priority of the remote
candidate is taken from the SDP of the peer. If the candidate does
not appear there, then the check must have been a triggered check to
a new remote candidate. In that case, the priority is taken as the
value of the PRIORITY attribute in the Binding Request which
triggered the check that just completed. The pair is then added to
the VALID LIST.
7.1.2.2.4. Updating the Nominated Flag
If the agent was a controlling agent, and it had included a USE-
CANDIDATE attribute in the Binding Request, the valid pair generated
from that check has its nominated flag set to true. This flag
indicates that this candidate should be used for media if it is the
highest priority one amongst those whose nominated flag is set. This
may conclude ICE processing for this media stream or all media
streams; see Section 8.
If the agent is the controlled agent, the response may result in the
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valid pair having its nominated flag set. See Section 7.2.1.4 for
the procedure.
7.2. Server Procedures
An agent MUST be prepared to receive a Binding Request on the base of
each candidate it included in its most recent offer or answer.
Receipt of a Binding Request on a base is an indication that the
connectivity check usage applies to the request.
The agent MUST use a short term credential to authenticate the
request and perform a message integrity check. The agent MUST accept
a credential if the username consists of two values separated by a
colon, where the first value is equal to the username fragment
generated by the agent in an offer or answer for a session in-
progress, and the MESSAGE-INTEGRITY is the output of a hash of the
password and the STUN packet's contents. It is possible (and in fact
very likely) that an offeror will receive a Binding Request prior to
receiving the answer from its peer. However, the request can be
processed without receiving this answer, and a response generated.
By doing this, ICE processing completes faster.
If the agent is using Diffserv Codepoint markings [27] in its media
packets, it SHOULD apply those same markings to its responses to
Binding Requests.
7.2.1. Additional Procedures for Full Implementations
This subsection defines the additional server procedures applicable
to full implementations when generating a successful response to a
Binding Request.
7.2.1.1. Computing Mapped Address
For requests being received on a relayed candidate, the source
transport address used for STUN processing (namely, generation of the
XOR-MAPPED-ADDRESS attribute) is the transport address as seen by the
relay. That source transport address will be present in the REMOTE-
ADDRESS attribute of a STUN Data Indication message, if the Binding
Request was delivered through a Data Indication (a STUN relay
delivers packets encapsulated in a Data Indication when no active
destination is set). If the Binding Request was not encapsulated in
a Data Indication, that source address is equal to the current active
destination for the STUN relay session.
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Internet-Draft ICE March 20077.2.1.2. Learning Peer Reflexive Candidates
If the source transport address of the request does not match any
existing remote candidates, it represents a new peer reflexive remote
candidate. The full-mode agent gives the candidate a priority equal
to the PRIORITY attribute from the request. The type of the
candidate is equal to peer reflexive. Its foundation is set to an
arbitrary value, different from the foundation for all other remote
candidates. If any subsequent offer/answer exchanges contain this
peer reflexive candidate in the SDP, it will signal the actual
foundation for the candidate. This candidate is then added to the
list of remote candidates. However, the agent does not pair this
candidate with any local candidates.
7.2.1.3. Triggered Checks
Next, the agent constructs a pair whose local candidate is equal to
the transport address on which the STUN request was received, and a
remote candidate equal to the source transport address where the
request came from (which may be peer-reflexive remote candidate that
was just learned). Since both candidates are known to the agent, it
can obtain their priorities and compute the candidate pair priority.
This pair is then looked up in the check list. There can be one of
several outcomes:
o If the pair is already on the check list:
* If the state of that pair is Waiting or Frozen, its state is
changed to In-Progress and a check for that pair is performed
immediately. This is called a triggered check.
* If the state of that pair is In-Progress, the agent SHOULD
generate an immediate retransmit of the Binding Request for the
check in progress. This is to facilitate rapid completion of
ICE when both agents are behind NAT.
* If the state of that pair is Failed or Succeeded, no triggered
check is sent.
o If the pair is not already on the check list:
* The pair is inserted into the check list based on its priority
* Its state is set to In-Progress
* A triggered check for that pair is performed immediately.
If a triggered check is to be generated, it is constructed and
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processed as described in Section 7.1.1. These procedures require
the agent to know the transport address, username fragment and
password for the peer. The username fragment for the remote
candidate is equal to the part after the colon of the USERNAME in the
Binding Request that was just received. Using that username
fragment, the agent can check the SDP messages received from its peer
(there may be more than one in cases of forking), and find this
username fragment. The corresponding password is then selected. If
agent has not yet received the username in an SDP (a likely case for
the offerer in the initial offer/answer exchange), it MUST wait for
the SDP to be received (since it won't have its peer's ICE password
without it), and then proceed with the triggered check.
7.2.1.4. Updating the Nominated Flag
If the Binding Request received by the agent had the USE-CANDIDATE
attribute set, and the agent is in the controlled role, the agent
looks at the state of the pair computed in Section 7.2.1.3:
o If the state of this pair is succeeded, it means that the check
generated by this pair produced a successful response. This would
have caused the agent to construct a valid pair when that success
response was received (see Section 7.1.2.2.3). The agent now sets
the nominated flag in the valid pair to true. This may end ICE
processing for this media stream; see Section 8.
o If the state of this pair is In-Progress, if its check produces a
successful result, the resulting valid pair has its nominated flag
set when the response arrives. This may end ICE processing for
this media stream when it arrives; see Section 8.
7.2.2. Additional Procedures for Lite Implementations
If the check that was just received contained a USE-CANDIDATE
attribute, the agent constructs a candidate pair whose local
candidate is equal to the transport address on which the request was
received, and whose remote candidate is equal to the source transport
address of the request that was received. This candidate pair is
assigned an arbitrary priority, and placed into a list of valid
candidates pair for that component of that media stream, called the
valid list. The agent sets the nominated flag for that pair to true.
ICE processing is considered complete for a media stream if the valid
list contains a candidate pair for each component.
8. Concluding ICE Processing
The processing rules in this section apply only to full
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implementations. Concluding ICE involves nominating pairs by the
controlling agent and updating of state machinery
8.1. Nominating Pairs
The controlling agent nominates pairs to be selected by ICE by using
one of two techniques: regular nomination or aggressive nomination.
If its peer has a lite implementation, an agent MUST use a regular
nomination algorithm. If its peer is using ICE options (present in
an ice-options attribute from the peer) that the agent does not
understand, the agent MUST use a regular nomination algorithm. If
its peer is a full implementation and isn't using any ICE options or
is using ICE options understood by the agent, the agent MAY use
either the aggressive or the regular nomination algorithm. However,
the regular algorithm is RECOMMENDED since it provides greater
stability.
8.1.1. Regular Nomination
With regular nomination, the agent lets some number of checks
complete, each of which omit the USE-CANDIDATE attribute. Once one
or more checks complete successfully for a component of a media
stream, valid pairs are generated and added to the valid list. The
agent lets the checks continue until some stopping criteria is met,
and then picks amongst the valid pairs based on an evaluation
criteria. The criteria for stopping the checks and for evaluating
the valid pairs is entirely a matter of local optimization.
When the controlling agent selects the valid pair, it repeats the
check that produced this valid pair, this time with the USE-CANDIDATE
attribute. This check will succeed (since the previous did), causing
the nominated flag of that and only that pair to be set.
Consequently, there will be only a single nominated pair in the valid
list, and when the state of the check list moves to completed, that
exact pair is selected by ICE for sending and receiving media.
Regular nomination provides the most flexibility, since the agent has
control over the stopping and selection criteria for checks. The
only requirement is that the agent MUST eventually pick one and only
one candidate pair and generate a check for that pair with the USE-
CANDIDATE attribute present. Regular nomination also improves ICE's
resilience to variations in implementation (see Section 14. Regular
nomination is also more stable, allowing both agents to converge on a
single pair for media without any transient selections, which can
happen with the aggressive algorithm. The drawback of regular
nomination is that it is guaranteed to increase latencies because it
requires an additional check to be done.
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Internet-Draft ICE March 20078.1.2. Aggressive Nomination
With aggressive nomination, the controlling agent includes the USE-
CANDIDATE attribute in every check it sends. Once the first check
for a component succeeds, it will be added to the valid list, have
its nominated flag set, and then cause ICE processing to cease for
this check list. However, because the agent included the USE-
CANDIDATE attribute in all of its checks, another check may yet
complete, causing another valid pair to have its nominated flag set.
ICE always selects the highest priority nominated candidate pair from
the valid list as the one used for media. Consequently, the selected
pair may actually change briefly as ICE checks complete, resulting in
a set of transient selections until it stabilizes.
8.2. Updating States
For both controlling and controlled agents, the state of ICE
processing depends on the presence of nominated candidate pairs in
the valid list:
o If there are no nominated pairs in the valid list for a media
stream, ICE processing continues.
o If there is at least one nominated pair in the valid list:
* The agent MUST remove all Waiting and Frozen pairs in the check
list for the same component as the nominated pairs for that
media stream
* If an In-Progress pair in the check list is for the same
component as a nominated pair, the agent SHOULD cease
retransmissions for its check if its pair priority is lower
than the lowest priority nominated pair for that component
o Once there is at least one nominated pair in the valid list for
every component of at least one media stream:
* The agent MUST change the state of processing for its check
list for that media stream to Completed.
* The agent MUST continue to respond to any checks it may still
receive for that media stream, and MUST perform triggered
checks if required by the processing of Section 7.2.
* The agent MAY begin transmitting media for this media stream as
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o Once there is at least one nominated pair in the valid list for
each component of each media stream:
* The agent sets the state of ICE processing overall to
Completed.
* If an agent is controlling, it examines the highest priority
nominated candidate pair for each component of each media
stream. If any of those candidate pairs differ from the
default candidate pairs in the most recent offer/answer
exchange, the controlling agent MUST generate an updated offer
as described in Section 9. If the controlling agent is using
an aggressive nomination algorithm, this may result in several
updated offers as the pairs selected for media change. An
agent MAY delay sending the offer for a brief interval (one
second is RECOMMENDED) in order to allow the selected pairs to
stabilize.
9. Subsequent Offer/Answer Exchanges
Either agent MAY generate a subsequent offer at any time allowed by
RFC 3264 [4]. The rules in Section 8 will cause the controlling
agent to send an updated offer at the conclusion of ICE processing
when ICE has selected different candidate pairs from the default
pairs. This section defines rules for construction of subsequent
offers and answers.
9.1. Generating the Offer9.1.1. Procedures for All Implementations9.1.1.1. ICE Restarts
An agent MAY restart ICE processing for an existing media stream. An
ICE restart, as the name implies, will cause all previous state of
ICE processing to be flushed and checks to start anew. The only
difference between an ICE restart and a brand new media session is
that, during the restart, media can continue to be sent to the
previously validated pair.
An agent MUST restart ICE for a media stream if:
o The offer is being generated for the purposes of changing the
target of the media stream. In other words, if an agent wants to
generated an updated offer which, had ICE not been in use, would
result in a new value for the destination of a media component.
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o An agent is changing its implementation level. This typically
only happens in third party call control use cases, where the
entity performing the signaling is not the entity receiving the
media, and it has changed the target of media mid-session to
another entity that has a different ICE implementation.
These rules imply that setting the IP address in the c line to
0.0.0.0 will cause an ICE restart. Consequently, ICE implementations
MUST NOT utilize this mechanism for call hold, and instead MUST use
a=inactive and a=sendonly as described in [4]
To restart ICE, an agent MUST change both the ice-pwd and the ice-
ufrag for the media stream in an offer. Note that it is permissible
to use a session-level attribute in one offer, but to provide the
same ice-pwd or ice-ufrag as a media-level attribute in a subsequent
offer. This is not a change in password, just a change in its
representation, and does not cause an ICE restart.
An agent sets the rest of the fields in the SDP for this media stream
as it would in an initial offer of this media stream (see
Section 4.3). Consequently, the set of candidates MAY include some,
none, or all of the previous candidates for that stream and MAY
include a totally new set of candidates gathered as described in
Section 4.1.1.
9.1.1.2. Removing a Media Stream
If an agent removes a media stream by setting its port to zero, it
MUST NOT include any candidate attributes for that media stream and
SHOULD NOT include any other ICE-related attributes defined in
Section 15 for that media stream.
9.1.1.3. Adding a Media Stream
If an agent wishes to add a new media stream, it sets the fields in
the SDP for this media stream as if this was an initial offer for
that media stream (see Section 4.3). This will cause ICE processing
to begin for this media stream.
9.1.2. Procedures for Full Implementations
This section describes additional procedures for full
implementations, covering existing media streams.
The username fragments, password, and implementation level MUST
remain the same as used previously. If an agent needs to change one
of these it MUST restart ICE for that media stream.
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Additional behavior depends on the state ICE processing for that
media stream.
9.1.2.1. Existing Media Streams with ICE Running
If an agent generates an updated offer including media stream that
was previously established, and for which ICE checks are in the
Running state, the agent follows the procedures defined here.
An agent MUST include candidate attributes for all local candidates
it had signaled previously for that media stream. The properties of
that candidate as signaled in SDP - the priority, foundation, type
and related transport address SHOULD remain the same. The IP
address, port and transport protocol, which fundamentally identify
that candidate, MUST remain the same (if they change, it would be a
new candidate). The component ID MUST remain the same. The agent
MAY include additional candidates it did not offer previously, but
which it has gathered since the last offer/answer exchange, including
peer reflexive candidates.
The agent MAY change the default destination for media. As with
initial offers, there MUST be a set of candidate attributes in the
offer matching this default destination.
9.1.2.2. Existing Media Streams with ICE Completed
If an agent generates an updated offer including media stream that
was previously established, and for which ICE checks are in the
Completed state, the agent follows the procedures defined here.
The default destination for media (i.e., the values of the IP
addresses and ports in the m and c line used for that media stream)
MUST be the local candidate from the highest priority nominated pair
in the valid list for each component. This "fixes" the default
destination for media to equal the destination ICE has selected for
media.
The agent MUST include a candidate attributes for candidates matching
the default destination for each component of the media stream, and
MUST NOT include any other candidates.
In addition, if the agent is controlling, it MUST include the
a=remote-candidates attribute for each media stream whose check list
is in the Completed state. The attribute contains the remote
candidates from the highest priority nominated pair in the valid list
for each component of that media stream. It is needed to avoid a
race condition whereby the controlling agent chooses its pairs, but
the updated offer beats the connectivity checks to the controlled
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agent, which doesn't even know these pairs are valid, let alone
selected. See Appendix B.6 for elaboration on this race condition.
9.1.3. Procedures for Lite Implementations
This section describes procedures for lite implementations for
existing streams for which ICE is running.
A lite implementation MUST include its one and only candidate for
each component of each media stream in an a=candidate attribute in
any subsequent offer. This candidate is formed identically to the
procedures for initial offers, as described in Section 4.2.
The username fragments, password, and implementation level MUST
remain the same as used previously. If an agent needs to change one
of these it MUST restart ICE for that media stream.
9.2. Receiving the Offer and Generating an Answer9.2.1. Procedures for All Implementations
When receiving a subsequent offer within an existing session, an
agent MUST re-apply the verification procedures in Section 5.1
without regard to the results of verification from any previous
offer/answer exchanges. Indeed, it is possible that a previous
offer/answer exchange resulted in ICE not being used, but it is used
as a consequence of a subsequent exchange.
9.2.1.1. Detecting ICE Restart
If the offer contained a change in the a=ice-ufrag or a=ice-pwd
attributes compared to the previous SDP from the peer, it indicates
that ICE is restarting for this media stream. If all media streams
are restarting, than ICE is restarting overall.
If ICE is restarting for a media stream:
o The agent MUST change the a=ice-ufrag and a=ice-pwd attributes in
the answer.
o The agent MAY change its implementation level in the answer.
An agent sets the rest of the fields in the SDP for this media stream
as it would in an initial answer to this media stream (see
Section 4.3). Consequently, the set of candidates MAY include some,
none, or all of the previous candidates for that stream and MAY
include a totally new set of candidates gathered as described in
Section 4.1.1.
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Internet-Draft ICE March 20079.2.1.2. New Media Stream
If the offer contains a new media stream, the agent sets the fields
in the answer as if it had received an initial offer containing that
media stream (see Section 4.3). This will cause ICE processing to
begin for this media stream.
9.2.1.3. Removed Media Stream
If an offer contains a media stream whose port is zero, the agent
MUST NOT include any candidate attributes for that media stream in
its answer and SHOULD NOT include any other ICE-related attributes
defined in Section 15 for that media stream.
9.2.2. Procedures for Full Implementations
The username fragments, password, and implementation level MUST
remain the same as used previously. If an agent needs to change one
of these it MUST restart ICE for that media stream by generating an
offer; ICE cannot be restarted in an answer.
Additional behaviors depend on the state of ICE processing for that
media stream.
9.2.2.1. Existing Media Streams with ICE Running and no remote- candidates
If ICE is running for a media stream, and the offer for that media
stream lacked the remote-candidates attribute, the rules for
construction of the answer are identical to those for the offerer as
described in Section 9.1.2.1.
9.2.2.2. Existing Media Streams with ICE Completed and no remote- candidates
If ICE is Completed for a media stream, and the offer for that media
stream lacked the remote-candidates attribute, the rules for
construction of the answer are identical to those for the offerer as
described in Section 9.1.2.2, except that the answerer MUST NOT
include the a=remote-candidates attribute in the answer.
9.2.2.3. Existing Media Streams and remote-candidates
A controlled agent will receive an offer with the a=remote-candidates
attribute for a media stream when its peer has concluded ICE
processing for that media stream. This attribute is present in the
offer to deal with a race condition between the receipt of the offer,
and the receipt of the Binding Response which tells the answerer the
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candidate which will be selected by ICE. See Appendix B.6 for an
explanation of this race condition. Consequently, processing of an
offer with this attribute depends on the winner of the race.
The agent forms a candidate pair for each component of the media
stream by:
o Setting the remote candidate equal to the offerers default
destination for that component (e.g., the contents of the m and
c-lines for RTP, and the a=rtcp attribute for RTCP)
o Setting the local candidate equal to the transport address for
that same component in the a=remote-candidates attribute in the
offer.
The agent then sees if each of these candidate pairs are present in
the valid list. If a particular pair is not in the valid list, the
check has "lost" the race. Call such a pair a "losing pair".
The agent finds all the pairs in the check list whose remote
candidates equal the remote candidate in the losing pair:
o If none of the pairs are In-Progress, and at least one is Failed,
it is most likely that a network failure, such as a network
partition or serious packet loss, has occurred. The agent SHOULD
generate an answer for this media stream as if the remote-
candidates attribute had not been present, and then restart ICE
for this stream.
o If at least one of the pairs are In-Progress, the agent SHOULD
wait for those checks to complete, and as each completes, redo the
processing in this section until there are no losing pairs.
Once there are no losing pairs, the agent can generate the answer.
It MUST set the default destination for media to the candidates in
the remote-candidates attribute from the offer (each of which will
now be the local candidate of a candidate pair in the valid list).
It MUST include a candidate attribute in the answer for each
candidate in the remote-candidates attribute in the offer.
9.2.3. Procedures for Lite Implementations
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Internet-Draft ICE March 20079.3. Updating the Check and Valid Lists9.3.1. Procedures for Full Implementations9.3.1.1. ICE Restarts
The agent MUST remember the highest priority nominated pairs in the
Valid list for each component of the media stream, called the
previous selected pairs, prior to the restart. The agent will
continue to send media using these pairs, as described in
Section 11.1. Once these destinations are noted, the agent MUST
flush the valid and check lists, and then recompute the check list
and its states as described in Section 5.7.
9.3.1.2. New Media Stream
If the offer/answer exchange added a new media stream, the agent MUST
create a new check list for it (and an empty Valid list to start of
course), as described in Section 5.7.
9.3.1.3. Removed Media Stream
If the offer/answer exchange removed a media stream, or an answer
rejected an offered media stream, an agent MUST flush the Valid list
for that media stream. It MUST terminate any STUN transactions in
progress for that media stream. An agent MUST remove the check list
for that media stream and cancel any pending periodic checks for it.
9.3.1.4. ICE Continuing for Existing Media Stream
The valid list is not affected by an updated offer/answer exchange
unless ICE is restarting.
If an agent is in the Running state for that media stream, the check
list is updated (the check list is irrelevant if the state is
completed). To do that, the agent recomputes the check list using
the procedures described in Section 5.7. If a pair on the new check
list was also on the previous check list, and its state was Waiting,
In-Progress, Succeeded or Failed, its state is copied over.
Otherwise, its state is set to Frozen.
If none of the check lists are active (meaning that the pairs in each
check list are Frozen), the full-mode agent sets the first pair in
the check list for the first media stream to Waiting, and then sets
the state of all other pairs in that check list for the same
component ID and with the same foundation to Waiting as well.
Next, the agent goes through each check list, starting with the
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highest priority pair. If a pair has a state of Succeeded, and it
has a component ID of 1, then all Frozen pairs in the same check list
with the same foundation whose component IDs are not 1, have their
state set to Waiting. If, for a particular check list, there are
pairs for each component of that media stream in the Succeeded state,
the agent moves the state of all Frozen pairs for the first component
of all other media streams (and thus in different check lists) with
the same foundation to Waiting.
9.3.2. Procedures for Lite Implementations
If ICE is restarting for a media stream, the agent MUST start a new
Valid list for that media stream. It MUST remember the pairs in the
previous Valid list for each component of the media stream, called
the previous selected pairs, and continue to send media there as
described in Section 11.1.
10. Keepalives
All endpoints MUST send keepalives for each media session. These
keepalives serve the purpose of keeping NAT bindings alive for the
media session. These keepalives MUST be sent regardless of whether
the media stream is currently inactive, sendonly, recvonly or
sendrecv, and regardless of the presence or value of the bandwidth
attribute. These keepalives MUST be sent even if ICE is not being
utilized for the session at all. The keepalive SHOULD be sent using
a format which is supported by its peer. ICE endpoints allow for
STUN-based keepalives for UDP streams, and as such, STUN keepalives
MUST be used when an agent is communicating with a peer that supports
ICE. An agent can determine that its peer supports ICE by the
presence of a=candidate attributes for each media session. If the
peer does not support ICE, the choice of a packet format for
keepalives is a matter of local implementation. A format which
allows packets to easily be sent in the absence of actual media
content is RECOMMENDED. Examples of formats which readily meet this
goal are RTP No-Op [31] and RTP comfort noise [25]. If the peer
doesn't support any formats that are particularly well suited for
keepalives, an agent SHOULD send RTP packets with an incorrect
version number, or some other form of error which would cause them to
be discarded by the peer.
If there has been no packet sent on the candidate pair ICE is using
for a media component for Tr seconds (where packets include those
defined for the component (RTP or RTCP) and previous keepalives), an
agent MUST generate a keepalive on that pair. Tr SHOULD be
configurable and SHOULD have a default of 15 seconds. Alternatively,
if an agent has a dynamic way to discover the binding lifetimes of
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the intervening NATs, it can use that value to determine Tr.
If STUN is being used for keepalives, a STUN Binding Indication is
used [12]. The Binding Indication SHOULD NOT contain integrity
checks as the messages are simply discarded on receipt regardless of
contents. The Indication SHOULD NOT contain the PRIORITY or USE-
CANDIDATE attributes defined in this document. The Binding
Indication is sent using the same local and remote candidates that
are being used for media. An agent receiving a Binding Indication
MUST discard it silently. Though Binding Indications are used for
keepalives, an agent MUST be prepared to receive Binding Requests as
well. If a Binding Request is received, a response is generated as
discussed in [12], but there is no impact on ICE processing
otherwise.
An agent MUST begin the keepalive processing once ICE has selected
candidates for usage with media, or media begins to flow, whichever
happens first. Keepalives end once the session terminates or the
media stream is removed.
11. Media Handling11.1. Sending Media
Procedures for sending media differ for full and lite
implementations.
11.1.1. Procedures for Full Implementations
Agents always send media using a candidate pair, called the selected
candidate pair. An agent will send media to the remote candidate in
the selected pair (setting the destination address and port of the
packet equal to that remote candidate), and will send it from the
local candidate of the selected pair. When the local candidate is
server or peer reflexive, media is originated from the base. Media
sent from a relayed candidate is sent from the base through that
relay, using procedures defined in [13].
The selected pair for a component of a media stream is:
o empty if the state of the check list for that media stream is
Running, and there is no previous selected pair for that component
due to an ICE restart
o equal to the previous selected pair for a component of a media
stream if the state of the check list for that media stream is
Running, and there was a previous selected pair for that component
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due to an ICE restart
o equal to the highest priority nominated pair for that component in
the valid list if the state of the check list is Completed
If the selected pair for at least one component of a media stream is
empty, an agent MUST NOT send media for any component of that media
stream. If the selected pair for each component of a media stream
has a value, an agent MAY send media for all components of that media
stream.
Note that the selected pair for a component of a media stream may not
equal the default pair for that same component from the most recent
offer/answer exchange. When this happens, the selected pair is used
for media, not the default pair. When ICE first completes, if the
selected pairs aren't a match for the default pairs, the controlling
agent sends an updated offer/answer exchange to remedy this
disparity. However, until that updated offer arrives, there will not
be a match. Furthermore, in very unusual cases, the default
candidates in the updated offer/answer will not be a match.
11.1.2. Procedures for Lite Implementations
A lite implementation MUST NOT send media until it has a Valid list
that contains a candidate pair for each component of that media
stream. Once that happens, the agent MAY begin sending media
packets. To do that, it sends media to the remote candidate in the
pair (setting the destination address and port of the packet equal to
that remote candidate), and will send it from the local candidate.
11.1.3. Procedures for All Implementations
ICE has interactions with jitter buffer adaptation mechanisms. An
RTP stream can begin using one candidate, and switch to another one,
though this happens rarely with ICE. The newer candidate may result
in RTP packets taking a different path through the network - one with
different delay characteristics. As discussed below, agents are
encouraged to re-adjust jitter buffers when there are changes in
source or destination address of media packets. Furthermore, many
audio codecs use the marker bit to signal the beginning of a
talkspurt, for the purposes of jitter buffer adaptation. For such
codecs, it is RECOMMENDED that the sender set the marker bit [22]
when an agent switches transmission of media from one candidate pair
to another.
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ICE implementations MUST be prepared to receive media on each
component on any candidates provided for that component in the most
recent offer/answer exchange (in the case of RTP, this would include
both RTP and RTCP if candidates were provided for both).
It is RECOMMENDED that, when an agent receives an RTP packet with a
new source or destination IP address for a particular media stream,
that the agent re-adjust its jitter buffers.
RFC 3550 [22] describes an algorithm in Section 8.2 for detecting
SSRC collisions and loops. These algorithms are based, in part, on
seeing different source transport addresses with the same SSRC.
However, when ICE is used, such changes will sometimes occur as the
media streams switch between candidates. An agent will be able to
determine that a media stream is from the same peer as a consequence
of the STUN exchange that proceeds media transmission. Thus, if
there is a change in source transport address, but the media packets
come from the same peer agent, this SHOULD NOT be treated as an SSRC
collision.
12. Usage with SIP12.1. Latency Guidelines
ICE requires a series of STUN-based connectivity checks to take place
between endpoints. These checks start from the answerer on
generation of its answer, and start from the offerer when it receives
the answer. These checks can take time to complete, and as such, the
selection of messages to use with offers and answers can effect
perceived user latency. Two latency figures are of particular
interest. These are the post-pickup delay and the post-dial delay.
The post-pickup delay refers to the time between when a user "answers
the phone" and when any speech they utter can be delivered to the
caller. The post-dial delay refers to the time between when a user
enters the destination address for the user, and ringback begins as a
consequence of having successfully started ringing the phone of the
called party.
Two cases can be considered - one where the offer is present in the
initial INVITE, and one where it is in a response.
12.1.1. Offer in INVITE
To reduce post-dial delays, it is RECOMMENDED that the caller begin
gathering candidates prior to actually sending its initial INVITE.
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This can be started upon user interface cues that a call is pending,
such as activity on a keypad or the phone going offhook.
If an offer is received in an INVITE request, the answerer SHOULD
begin to gather its candidates on receipt of the offer and then
generate an answer in a provisional response once it has completed
that process. ICE requires that a provisional response with an SDP
be transmitted reliably. This can be done through the existing PRACK
mechanism [9], or through an optimization that is specific to ICE.
With this optimization, provisional responses containing an SDP
answer that begins ICE processing for one or more media streams can
be sent reliably without RFC 3264. To do this, the agent retransmits
the provisional response with th exponential backoff timers described
in RFC 3262. Retransmits MUST cease on receipt of a STUN Binding
Request for one of the media streams signaled in that SDP (because
receipt of a binding request indicates the offerer has received the
answer) or on transmission of a 2xx response. If no Binding Request
is received prior to the last retransmit, the agent does not consider
the session terminated. Despite the fact that the provisional
response will be delivered reliably, the rules for when an agent can
send an updated offer or answer do not change from those specified in
RFC 3262. Specifically, if the INVITE contained an offer, the same
answer appears in all of the 1xx and in the 2xx response to the
INVITE. Only after that 2xx has been sent can an updated offer/
answer exchange occur. This optimization SHOULD NOT be used if both
agents support PRACK. Note that the optimization is very specific to
provisional response carrying answers that start ICE processing; it
is not a general technique for 1xx reliability.
Alternatively, an agent MAY delay sending an answer until the 200 OK,
however this results in a poor user experience and is NOT
RECOMMENDED.
Once the answer has been sent, the agent SHOULD begin its
connectivity checks. Once candidate pairs for each component of a
media stream enter the valid list, the answerer can begin sending
media on that media stream.
However, prior to this point, any media that needs to be sent towards
the caller (such as SIP early media [26] MUST NOT be transmitted.
For this reason, implementations SHOULD delay alerting the called
party until candidates for each component of each media stream have
entered the valid list. In the case of a PSTN gateway, this would
mean that the setup message into the PSTN is delayed until this
point. Doing this increases the post-dial delay, but has the effect
of eliminating 'ghost rings'. Ghost rings are cases where the called
party hears the phone ring, picks up, but hears nothing and cannot be
heard. This technique works without requiring support for, or usage
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of, preconditions [6], since its a localized decision. It also has
the benefit of guaranteeing that not a single packet of media will
get clipped, so that post-pickup delay is zero. If an agent chooses
to delay local alerting in this way, it SHOULD generate a 180
response once alerting begins.
12.1.2. Offer in Response
In addition to uses where the offer is in an INVITE, and the answer
is in the provisional and/or 200 OK response, ICE works with cases
where the offer appears in the response. In such cases, which are
common in third party call control [18], ICE agents SHOULD generate
their offers in a reliable provisional response (which MUST utilize
RFC 3262), and not alert the user on receipt of the INVITE. The
answer will arrive in a PRACK. This allows for ICE processing to
take place prior to alerting, so that there is no post-pickup delay,
at the expense of increased call setup delays. Once ICE completes,
the callee can alert the user and then generate a 200 OK when they
answer. The 200 OK would contain no SDP, since the offer/answer
exchange has completed.
Alternatively, agents MAY place the offer in a 2xx instead (in which
case the answer comes in the ACK). When this happens, the callee
will alert the user on receipt of the INVITE, and the ICE exchanges
will take place only after the user answers. This has the effect of
reducing call setup delay, but can cause substantial post-pickup
delays and media clipping.
12.2. SIP Option Tags and Media Feature Tags
[14] specifies a SIP option tag and media feature tag for usage with
ICE. ICE implementations using SIP SHOULD support this
specification, which uses a feature tag in registrations to
facilitate interoperability through intermediaries.
12.3. Interactions with Forking
ICE interacts very well with forking. Indeed, ICE fixes some of the
problems associated with forking. Without ICE, when a call forks and
the caller receives multiple incoming media streams, it cannot
determine which media stream corresponds to which callee.
With ICE, this problem is resolved. The connectivity checks which
occur prior to transmission of media carry username fragments, which
in turn are correlated to a specific callee. Subsequent media
packets which arrive on the same candidate pair as the connectivity
check will be associated with that same callee. Thus, the caller can
perform this correlation as long as it has received an answer.
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Quality of Service (QoS) preconditions, which are defined in RFC 3312
[6] and RFC 4032 [7], apply only to the transport addresses listed as
the default targets for media in an offer/answer. If ICE changes the
transport address where media is received, this change is reflected
in an updated offer which changes the default destination for media
to match ICE's selection. As such, it appears like any other re-
INVITE would, and is fully treated in RFC 3312 and 4032, which apply
without regard to the fact that the destination for media is changing
due to ICE negotiations occurring "in the background".
Indeed, an agent SHOULD NOT indicate that Qos preconditions have been
met until the checks have completed and selected the candidate pairs
to be used for media.
ICE also has (purposeful) interactions with connectivity
preconditions [30]. Those interactions are described there. Note
that the procedures described in Section 12.1 describe their own type
of "preconditions", albeit with less functionality than those
provided by the explicit preconditions in [30].
12.5. Interactions with Third Party Call Control
ICE works with Flows I, III and IV as described in [18]. Flow I
works without the controller supporting or being aware of ICE. Flow
IV will work as long as the controller passes along the ICE
attributes without alteration. Flow II is fundamentally incompatible
with ICE; each agent will believe itself to be the answerer and thus
never generate a re-INVITE.
The flows for continued operation, as described in Section 7 of RFC3725, require additional behavior of ICE implementations to support.
In particular, if an agent receives a mid-dialog re-INVITE that
contains no offer, it MUST restart ICE for each media stream and go
through the process of gathering new candidates. Furthermore, that
list of candidates SHOULD include the ones currently being used for
media.
13. Usage with ANATRFC 4091 [11] defines a mechanism for indicating that an agent can
support both IPv4 and IPv6 for a media stream, and it does so by
including two m-lines, one for v4, and one for v6. This is similar
to ICE, which allows for an agent to indicate multiple transport
addresses using the candidate attribute.
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However, ICE is not a replacement for ANAT. When an agent has a v4
and v6 interface and requires just a static choice of address - use
v6 if both support v6, else v4 - ANAT alone is used. If an agent
wishes the choice of v4 or v6 to be dynamic and depend on actual
verification of connectivity, an agent would use ANAT in concert with
ICE. To do that, The agent MUST include two media stream alternates,
one for v4 and one for v6, as defined in RFC 4091. In addition, the
agent MUST include a v4 candidate as a session attribute for the v4
stream alternate, and a v6 candidate as a session attribute of the v6
stream alternate. ICE will then perform its checks for each stream
alternate. The agent MUST order the ICE selected pairs for each
stream alternate based on their mid preference, and choose the
highest one. This means that if ICE doesn't select any pair for a
stream alternate (because, for example, no checks succeeded), the
agent will choose the next highest preference pair which was
selected. This allows v6 to be used if a v6 path can be verified,
but to fallback to v4 if it cannot be verified.
This extends naturally to multiple candidates for each alternate. An
agent MAY include multiple v4 candidates for the v4 stream alternate
and multiple v6 candidates for the v6 stream alternate. All of the
candidates for a v4 stream alternate MUST be v4, and all of the
candidates for a v6 stream alternate MUST be v6. This will cause ICE
to choose a v6 pair as long as one of the pairs works, else it will
fall back to v4.
Of course, an agent can use ICE with v4 and v6 candidates without
ANAT. In that mode, it would have just a single media stream, with a
default destination that is either v4 or v6. The candidates can
include both v4 and v6 candidates. This brings an agent the
flexibility of choosing a v4 candidate even if a v6 candidate
validates, perhaps due to differing path characteristics measured
dynamically by the agent. That kind of flexibility is not possible
when ANAT is used.
14. Extensibility Considerations
This specification makes very specific choices about how both agents
in a session coordinate to arrive at the set of candidate pairs that
are selected for media. It is anticipated that future specifications
will want to alter these algorithms, whether they are simple changes
like timer tweaks, or larger changes like a revamp of the priority
algorithm. When such a change is made, providing interoperability
between the two agents in a session is critical.
First, ICE provides the a=ice-options SDP attribute. Each extension
or change to ICE is associated with a token. When an agent
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supporting such an extension or change generates an offer or an
answer, it MUST include the token for that extension in this
attribute. This allows each side to know what the other side is
doing. This attribute MUST NOT be present if the agent doesn't
support any ICE extensions or changes.
At this time, no IANA registry or registration procedures are defined
for these option tags. At time of writing, it is unclear whether ICE
changes and extensions will be sufficiently common to warrrant a
registry.
One of the complications in achieving interoperability is that ICE
relies on a distributed algorithm running on both agents to converge
on an agreed set of candidate pairs. If the two agents run different
algorithms, it can be difficult to guarantee convergence on the same
candidate pairs. The regular nomination procedure described in
Section 8 eliminates some of the tight coordination by delegating the
selection algorithm completely to the controlling agent.
Consequently, when a controlling agent is communicating with a peer
that supports options it doesn't know about, the agent MUST run a
regular nomination algorithm. When regular nomination is used, ICE
will converge perfectly even when both agents use different pair
prioritization algorithms. One of the keys to such convergence are
triggered checks, which ensure that the nominated pair is validated
by both agents. Consequently, any future ICE enhancements MUST
preserve triggered checks.
ICE is also extensible to other media streams beyond RTP, and for
transport protocols beyond UDP. Extensions to ICE for non-RTP media
streams need to specify how many components they utilize, and assign
component IDS to them, starting at 1 for the most important component
ID. Specifications for new transport protocols must define how, if
at all, various steps in the ICE processing differ from UDP.
15. Grammar
This specification defines seven new SDP attributes - the
"candidate", "remote-candidates", "ice-lite", "ice-mismatch", "ice-
ufrag", "ice-pwd" and "ice-options" attributes.
15.1. "candidate" Attribute
The candidate attribute is a media-level attribute only. It contains
a transport address for a candidate that can be used for connectivity
checks.
The syntax of this attribute is defined using Augmented BNF as
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<transport>: indicates the transport protocol for the candidate.
This specification only defines UDP. However, extensibility is
provided to allow for future transport protocols to be used with
ICE, such as TCP or the Datagram Congestion Control Protocol
(DCCP) [32].
<foundation>: is composed of one or more <ice-char>. It is an
identifier that is equivalent for two candidates that are of the
same type, share the same base, and come from the same STUN
server. The foundation is used to optimize ICE performance in the
Frozen algorithm.
<component-id>: is a positive integer between 1 and 256 which
identifies the specific component of the media stream for which
this is a candidate. It MUST start at 1 and MUST increment by 1
for each component of a particular candidate. For media streams
based on RTP, candidates for the actual RTP media MUST have a
component ID of 1, and candidates for RTCP MUST have a component
ID of 2. Other types of media streams which require multiple
components MUST develop specifications which define the mapping of
components to component IDs. See Section 14 for additional
discussion on extending ICE to new media streams.
<priority>: is a positive integer between 1 and (2**32 - 1).
<cand-type>: encodes the type of candidate. This specification
defines the values "host", "srflx", "prflx" and "relay" for host,
server reflexive, peer reflexive and relayed candidates,
respectively. The set of candidate types is extensible for the
future.
<rel-addr> and <rel-port>: convey transport addresses related to the
candidate, useful for diagnostics and other purposes. <rel-addr>
and <rel-port> MUST be present for server reflexive, peer
reflexive and relayed candidates. If a candidate is server or
peer reflexive, <rel-addr> and <rel-port> is equal to the base for
that server or peer reflexive candidate. If the candidate is
relayed, <rel-addr> and <rel-port> is equal to the mapped address
in the Allocate Response that provided the client with that
relayed candidate (see Appendix B.3 for a discussion of its
purpose). If the candidate is a host candidate <rel-addr> and
<rel-port> MUST be omitted.
The candidate attribute can itself be extended. The grammar allows
for new name/value pairs to be added at the end of the attribute. An
implementation MUST ignore any name/value pairs it doesn't
understand.
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The syntax of the "remote-candidates" attribute is defined using
Augmented BNF as defined in RFC 4234 [8]. The remote-candidates
attribute is a media level attribute only.
remote-candidate-att = "remote-candidates" ":" remote-candidate
0*(SP remote-candidate)
remote-candidate = component-ID SP connection-address SP port
The attribute contains a connection-address and port for each
component. The ordering of components is irrelevant. However, a
value MUST be present for each component of a media stream. This
attribute MUST be included in an offer by a controlling agent for a
media stream that is Completed, and MUST NOT be included in any other
case.
15.3. "ice-lite" and "ice-mismatch" Attributes
The syntax of the "ice-lite" and "ice-mismatch" attributes, both of
which are flags, is:
ice-lite = "ice-lite"
ice-mismatch = "ice-mismatch"
"ice-lite" is a session level attribute only, and indicates that an
agent is a lite implementation. "ice-mismatch" is a media level
attribute only, and when present in an answer, indicates that the
offer arrived with a default destination for a media component that
didn't have a corresponding candidate attribute.
15.4. "ice-ufrag" and "ice-pwd" Attributes
The "ice-ufrag" and "ice-pwd" attributes convey the username fragment
and password used by ICE for message integrity. Their syntax is:
ice-pwd-att = "ice-pwd" ":" password
ice-ufrag-att = "ice-ufrag" ":" ufrag
password = 22*ice-char
ufrag = 4*ice-char
The "ice-pwd" and "ice-ufrag" attributes can appear at either the
session-level or media-level. When present in both, the value in the
media-level takes precedence. Thus, the value at the session level
is effectively a default that applies to all media streams, unless
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overriden by a media-level value. Whether present at the session or
media level, there MUST be an ice-pwd and ice-ufrag attribute for
each media stream. If two media streams have identical ice-ufrag's,
they MUST have identical ice-pwd's.
The ice-ufrag and ice-pwd attributes MUST be chosen randomly at the
beginning of a session. The ice-ufrag attribute MUST contain at
least 24 bits of randomness, and the ice-pwd attribute MUST contain
at least 128 bits of randomness. This means that the ice-ufrag
attribute will be at least 4 characters long, and the ice-pwd at
least 22 characters long, since the grammar for these attributes
allows for 6 bits of randomness per character. The attributes MAY be
longer than 4 and 22 characters respectively, of course.
15.5. "ice-options> Attribute
The "ice-options" attribute is a session level attribute. It
contains a series of tokens which identify the options supported by
the agent. Its grammar is:
ice-options = "ice-options" ":" ice-option-tag
0*(SP ice-option-tag)
ice-option-tag = 1*ice-char
16. Example
The example is based on the simplified topology of Figure 15.
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+-----+
| |
|STUN |
| Srvr|
+-----+
|
+---------------------+
| |
| Internet |
| |
| |
+---------------------+
| |
| |
+---------+ |
| NAT | |
+---------+ |
| |
| |
| |
+-----+ +-----+
| | | |
| L | | R |
| | | |
+-----+ +-----+
Figure 15: Example Topology
Two agents, L and R, are using ICE. Both are full-mode ICE
implementations. Both agents have a single IPv4 interface. For
agent L, it is 10.0.1.1 in private address space [28], and for agent
R, 192.0.2.1 on the public Internet. Both are configured with the
same STUN server (shown in this example for simplicity, although in
practice the agents do not need to use the same STUN server), which
is listening for STUN requests at an IP address of 192.0.2.2 and port
3478. This STUN server supports only the Binding Discovery usage;
relays are not used in this example. Agent L is behind a NAT, and
agent R is on the public Internet. The NAT has an endpoint
independent mapping property and an address dependent filtering
property. The public side of the NAT has an IP address of 192.0.2.3.
To facilitate understanding, transport addresses are listed using
variables that have mnemonic names. The format of the name is
entity-type-seqno, where entity refers to the entity whose interface
the transport address is on, and is one of "L", "R", "STUN", or
"NAT". The type is either "PUB" for transport addresses that are
public, and "PRIV" for transport addresses that are private.
Finally, seq-no is a sequence number that is different for each
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| |MA=$R-PUB-1 | |
| |---------------------------->|
| | | |RTP flows
Figure 16: Example Flow
First, agent L obtains a host candidate from its local interface (not
shown), and from that, sends a STUN Binding Request to the STUN
server to get a server reflexive candidate (messages 1-4). Recall
that the NAT has the address and port independent mapping property.
Here, it creates a binding of NAT-PUB-1 for this UDP request, and
this becomes the server reflexive candidate for RTP.
Agent L sets a type preference of 126 for the host candidate and 100
for the server reflexive. The local preference is 65535. Based on
this, the priority of the host candidate is 2130706178 and for the
server reflexive candidate is 1694498562. The host candidate is
assigned a foundation of 1, and the server reflexive, a foundation of
2. It chooses its server reflexive candidate as the default
candidate, and encodes it into the m and c lines. The resulting
offer (message 5) looks like (lines folded for clarity):
v=0
o=jdoe 2890844526 2890842807 IN IP4 $L-PRIV-1.IP
s=
c=IN IP4 $NAT-PUB-1.IP
t=0 0
a=ice-pwd:asd88fgpdd777uzjYhagZg
a=ice-ufrag:8hhY
m=audio $NAT-PUB-1.PORT RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=candidate:1 1 UDP 2130706178 $L-PRIV-1.IP $L-PRIV-1.PORT typ local
a=candidate:2 1 UDP 1694498562 $NAT-PUB-1.IP $NAT-PUB-1.PORT typ srflx raddr
$L-PRIV-1.IP rport $L-PRIV-1.PORT
The offer, with the variables replaced with their values, will look
like (lines folded for clarity):
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Since neither side indicated that they are lite, the agent which sent
the offer that began ICE processing (agent L) becomes the controlling
agent.
Agents L and R both pair up the candidates. They both initially have
two pairs. However, agent L will prune the pair containing its
server reflexive candidate, resulting in just one. At agent L, this
pair has a local candidate of $L_PRIV_1 and remote candidate of
$R_PUB_1, and has a candidate pair priority of 4.57566E+18 (note that
an implementation would represent this as a 64 bit integer so as not
to lose precision). At agent R, there are two pairs. The highest
priority has a local candidate of $R_PUB_1 and remote candidate of
$L_PRIV_1 and has a priority of 4.57566E+18, and the second has a
local candidate of $R_PUB_1 and remote candidate of $NAT_PUB_1 and
priority 3.63891E+18.
Agent R begins its connectivity check (message 9) for the first pair
(between the two host candidates). Since R is the controlled agent
for this session, the check omits the USE-CANDIDATE attribute. The
host candidate from agent L is private and behind a NAT, and thus
this check won't be successful, because the packet cannot be routed
from R to L.
When agent L gets the answer, it performs its one and only
connectivity check (messages 10-13). It implements the aggressive
nomination algorithm, and thus includes a USE-CANDIDATE attribute in
this check. Since the check succeeds, agent L creates a new pair,
whose local candidate is from the mapped address in the binding
response (NAT-PUB-1 from message 13) and whose remote candidate is
the destination of the request (R-PUB-1 from message 10). This is
added to the valid list. In addition, it is marked as selected since
the Binding Request contained the USE-CANDIDATE attribute. Since
there is a selected candidate in the Valid list for the one component
of this media stream, ICE processing for this stream moves into the
Completed state. Agent L can now send media if it so chooses.
Upon receipt of the STUN Binding Request from agent L (message 11),
agent R will generate its triggered check. This check happens to
match the next one on its check list - from its host candidate to
agent L's server reflexive candidate. This check (messages 14-17)
will succeed. Consequently, agent R constructs a new candidate pair
using the mapped address from the response as the local candidate
(R-PUB-1) and the destination of the request (NAT-PUB-1) as the
remote candidate. This pair is added to the Valid list for that
media stream. Since the check was generated in the reverse direction
of a check that contained the USE-CANDIDATE attribute, the candidate
pair is marked as selected. Consequently, processing for this stream
moves into the Completed state, and agent R can also send media.
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Internet-Draft ICE March 200717. Security Considerations
There are several types of attacks possible in an ICE system. This
section considers these attacks and their countermeasures.
17.1. Attacks on Connectivity Checks
An attacker might attempt to disrupt the STUN connectivity checks.
Ultimately, all of these attacks fool an agent into thinking
something incorrect about the results of the connectivity checks.
The possible false conclusions an attacker can try and cause are:
False Invalid: An attacker can fool a pair of agents into thinking a
candidate pair is invalid, when it isn't. This can be used to
cause an agent to prefer a different candidate (such as one
injected by the attacker), or to disrupt a call by forcing all
candidates to fail.
False Valid: An attacker can fool a pair of agents into thinking a
candidate pair is valid, when it isn't. This can cause an agent
to proceed with a session, but then not be able to receive any
media.
False Peer-Reflexive Candidate: An attacker can cause an agent to
discover a new peer reflexive candidate, when it shouldn't have.
This can be used to redirect media streams to a DoS target or to
the attacker, for eavesdropping or other purposes.
False Valid on False Candidate: An attacker has already convinced an
agent that there is a candidate with an address that doesn't
actually route to that agent (for example, by injecting a false
peer reflexive candidate or false server reflexive candidate). It
must then launch an attack that forces the agents to believe that
this candidate is valid.
Of the various techniques for creating faked STUN messages described
in [12], many are not applicable for the connectivity checks.
Compromises of STUN servers are not much of a concern, since the STUN
servers are embedded in endpoints and distributed throughout the
network. Thus, compromising the peer's embedded STUN server is
equivalent to comprimising the endpoint, and if that happens, far
more problematic attacks are possible than those against ICE.
Similarly, DNS attacks are usually irrelevant since STUN servers are
not typically discovered via DNS, they are normally signaled via IP
addresses embedded in SDP. If the SDP does contain an FQDN for a
host, then connectivity checks would be susceptible to the DNS
vulnerabilities described in [12]; however it is far more common to
use IP addresses. Injection of fake responses and relaying modified
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requests all can be handled in ICE with the countermeasures discussed
below.
To force the false invalid result, the attacker has to wait for the
connectivity check from one of the agents to be sent. When it is,
the attacker needs to inject a fake response with an unrecoverable
error response, such as a 600. However, since the candidate is, in
fact, valid, the original request may reach the peer agent, and
result in a success response. The attacker needs to force this
packet or its response to be dropped, through a DoS attack, layer 2
network disruption, or other technique. If it doesn't do this, the
success response will also reach the originator, alerting it to a
possible attack. Fortunately, this attack is mitigated completely
through the STUN message integrity mechanism. The attacker needs to
inject a fake response, and in order for this response to be
processed, the attacker needs the password. If the offer/answer
signaling is secured, the attacker will not have the password.
Forcing the fake valid result works in a similar way. The agent
needs to wait for the Binding Request from each agent, and inject a
fake success response. The attacker won't need to worry about
disrupting the actual response since, if the candidate is not valid,
it presumably wouldn't be received anyway. However, like the fake
invalid attack, this attack is mitigated completely through the STUN
message integrity and offer/answer security techniques.
Forcing the false peer reflexive candidate result can be done either
with fake requests or responses, or with replays. We consider the
fake requests and responses case first. It requires the attacker to
send a Binding Request to one agent with a source IP address and port
for the false candidate. In addition, the attacker must wait for a
Binding Request from the other agent, and generate a fake response
with a XOR-MAPPED-ADDRESS attribute containing the false candidate.
Like the other attacks described here, this attack is mitigated by
the STUN message integrity mechanisms and secure offer/answer
exchanges.
Forcing the false peer reflexive candidate result with packet replays
is different. The attacker waits until one of the agents sends a
check. It intercepts this request, and replays it towards the other
agent with a faked source IP address. It must also prevent the
original request from reaching the remote agent, either by launching
a DoS attack to cause the packet to be dropped, or forcing it to be
dropped using layer 2 mechanisms. The replayed packet is received at
the other agent, and accepted, since the integrity check passes (the
integrity check cannot and does not cover the source IP address and
port). It is then responded to. This response will contain a XOR-
MAPPED-ADDRESS with the false candidate, and will be sent to that
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false candidate. The attacker must then receive it and relay it
towards the originator.
The other agent will then initiate a connectivity check towards that
false candidate. This validation needs to succeed. This requires
the attacker to force a false valid on a false candidate. Injecting
of fake requests or responses to achieve this goal is prevented using
the integrity mechanisms of STUN and the offer/answer exchange.
Thus, this attack can only be launched through replays. To do that,
the attacker must intercept the check towards this false candidate,
and replay it towards the other agent. Then, it must intercept the
response and replay that back as well.
This attack is very hard to launch unless the attacker is identified
by the fake candidate. This is because it requires the attacker to
intercept and replay packets sent by two different hosts. If both
agents are on different networks (for example, across the public
Internet), this attack can be hard to coordinate, since it needs to
occur against two different endpoints on different parts of the
network at the same time.
If the attacker themself is identified by the fake candidate the
attack is easier to coordinate. However, if SRTP is used [23], the
attacker will not be able to play the media packets, they will only
be able to discard them, effectively disabling the media stream for
the call. However, this attack requires the agent to disrupt packets
in order to block the connectivity check from reaching the target.
In that case, if the goal is to disrupt the media stream, its much
easier to just disrupt it with the same mechanism, rather than attack
ICE.
17.2. Attacks on Address Gathering
ICE endpoints make use of STUN for gathering candidates from a STUN
server in the network. This is corresponds to the Binding Discovery
usage of STUN described in [12]. As a consequence, the attacks
against STUN itself that are described in that specification can
still be used against the binding discovery usage when utilized with
ICE.
However, the additional mechanisms provided by ICE actually
counteract such attacks, making binding discovery with STUN more
secure when combined with ICE than without ICE.
Consider an attacker which is able to provide an agent with a faked
mapped address in a STUN Binding Request that is used for address
gathering. This is the primary attack primitive described in [12].
This address will be used as a server reflexive candidate in the ICE
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exchange. For this candidate to actually be used for media, the
attacker must also attack the connectivity checks, and in particular,
force a false valid on a false candidate. This attack is very hard
to launch if the false address identifies a fourth party (neither the
offerer, answerer, or attacker), since it requires attacking the
checks generated by each agent in the session, and is prevented by
SRTP if it identifies the attacker themself.
If the attacker elects not to attack the connectivity checks, the
worst it can do is prevent the server reflexive candidate from being
used. However, if the peer agent has at least one candidate that is
reachable by the agent under attack, the STUN connectivity checks
themselves will provide a peer reflexive candidate that can be used
for the exchange of media. Peer reflexive candidates are generally
preferred over server reflexive candidates. As such, an attack
solely on the STUN address gathering will normally have no impact on
a session at all.
17.3. Attacks on the Offer/Answer Exchanges
An attacker that can modify or disrupt the offer/answer exchanges
themselves can readily launch a variety of attacks with ICE. They
could direct media to a target of a DoS attack, they could insert
themselves into the media stream, and so on. These are similar to
the general security considerations for offer/answer exchanges, and
the security considerations in RFC 3264 [4] apply. These require
techniques for message integrity and encryption for offers and
answers, which are satisfied by the SIPS mechanism [3] when SIP is
used. As such, the usage of SIPS with ICE is RECOMMENDED.
17.4. Insider Attacks
In addition to attacks where the attacker is a third party trying to
insert fake offers, answers or stun messages, there are several
attacks possible with ICE when the attacker is an authenticated and
valid participant in the ICE exchange.
17.4.1. The Voice Hammer Attack
The voice hammer attack is an amplification attack. In this attack,
the attacker initiates sessions to other agents, and maliciously
includes the IP address and port of a DoS target as the destination
for media traffic signaled in the SDP. This causes substantial
amplification; a single offer/answer exchange can create a continuing
flood of media packets, possibly at high rates (consider video
sources). This attack is not specific to ICE, but ICE can help
provide remediation.
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Specifically, if ICE is used, the agent receiving the malicious SDP
will first perform connectivity checks to the target of media before
sending media there. If this target is a third party host, the
checks will not succeed, and media is never sent.
Unfortunately, ICE doesn't help if its not used, in which case an
attacker could simply send the offer without the ICE parameters.
However, in environments where the set of clients are known, and
limited to ones that support ICE, the server can reject any offers or
answers that don't indicate ICE support.
17.4.2. STUN Amplification Attack
The STUN amplification attack is similar to the voice hammer.
However, instead of voice packets being directed to the target, STUN
connectivity checks are directed to the target. The attacker sends
an offer with a large number of candidates, say 50. The answerer
receives the offer, and starts its checks, which are directed at the
target, and consequently, never generate a response. The answerer
will start a new connectivity check every 20ms, and each check is a
STUN transaction consisting of 7 transmissions of a message 65 bytes
in length (plus 28 bytes for the IP/UDP header) that runs for 7.9
seconds, for a total of 58 bytes/second per transaction on average.
In the worst case, there can be 395 transactions in progress at once
(7.9 seconds divided by 20ms), for a total of 182 kbps, just for STUN
requests.
It is impossible to eliminate the amplification, but the volume can
be reduced through a variety of heuristics. Agents SHOULD limit the
total number of connectivity checks they perform to 100.
Additionally, agents MAY limit the number of candidates they'll
accept in an offer or answer.
17.5. Interactions with Application Layer Gateways and SIP
Application Layer Gateways (ALGs) are functions present in a NAT
device which inspect the contents of packets and modify them, in
order to facilitate NAT traversal for application protocols. Session
Border Controllers (SBC) are close cousins of ALGs, but are less
transparent since they actually exist as application layer SIP
intermediaries. ICE has interactions with SBCs and ALGs.
If an ALG is SIP aware but not ICE aware, ICE will work through it as
long as the ALG correctly modifies the SDP. In this case, correctly
means that the ALG does not modify the m and c lines or the rtcp
attribute if they contain external addresses. If they contain
internal addresses, the modification depends on the state of the ALG.
If the ALG already has a binding established that maps an external
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port to an internal IP address and port in m and c lines or rtcp
attribute , the ALG uses that binding instead of creating a new one.
Unfortunately, many ALG are known to work poorly in these corner
cases. ICE does not try to work around broken ALGs, as this is
outside the scope of its functionality. ICE can help diagnose these
conditions, which often show up as a mismatch between the set of
candidates and the m and c lines and rtcp attributes. The ice-
mismatch attribute is used for this purpose.
ICE works best through ALGs when the signaling is run over TLS. This
prevents the ALG from manipulating the SDP messages and interfering
with ICE operation. Implementations which are expected to be
deployed behind ALGs SHOULD provide for TLS transport of the SDP.
If an SBC is SIP aware but not ICE aware, the result depends on the
behavior of the SBC. If it is acting as a proper Back-to-Back User
Agent (B2BUA), the SBC will remove any SDP attributes it doesn't
understand, including the ICE attributes. Consequently, the call
will appear to both endpoints as if the other side doesn't support
ICE. This will result in ICE being disabled, and media flowing
through the SBC, if the SBC has requested it. If, however, the SBC
passes the ICE attributes without modification, yet modifies the
default destination for media (contained in the m and c lines and
rtcp attribute), this will be detected as an ICE mismatch, and ICE
processing is aborted for the call. It is outside of the scope of
ICE for it to act as a tool for "working around" SBCs. If one is
present, ICE will not be used and the SBC techniques take precedence.
18. Definition of Connectivity Check Usage
STUN [12] requires that new usages provide a specific set of
information as part of their formal definition. This section meets
the requirements spelled out there.
18.1. Applicability
This STUN usage provides a connectivity check between two peers
participating in an offer/answer exchange. This check serves to
validate a pair of candidates for usage of exchange of media.
Connectivity checks also allow agents to discover reflexive
candidates towards their peers, called peer reflexive candidates.
Finally, this usage allows a Binding Indication to be used to keep
NAT bindings alive.
It is fundamental to this STUN usage that the addresses and ports
used for media are the same ones used for the Binding Requests and
responses. Consequently, it will be necessary to demultiplex STUN
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traffic from applications on that same port (e.g., RTP or RTCP).
This demultiplexing is done using the techniques described in [12].
18.2. Client Discovery of Server
The client does not follow the DNS-based procedures defined in [12].
Rather, the remote candidate of the check to be performed is used as
the transport address of the STUN server. Note that the STUN server
is a logical entity, and is not a physically distinct server in this
usage.
18.3. Server Determination of Usage
The server is aware of this usage because it signaled transport
addresses in its candidates on which it expects to receive STUN
packets. Consequently, any STUN packets received on the base of a
candidate offered in SDP will be for the connectivity check usage.
18.4. New Requests or Indications
This usage does not define any new message types.
18.5. New Attributes
This usage defines two new attributes, PRIORITY and USE-CANDIDATE.
The PRIORITY attribute indicates the priority that is to be
associated with a peer reflexive candidate, should one be discovered
by this check. It is a 32 bit unsigned integer, and has an attribute
value of 0x0024.
The USE-CANDIDATE attribute indicates that the candidate pair
resulting from this check should be used for transmission of media.
The attribute has no content (the Length field of the attribute is
zero); it serves as a flag. It has an attribute value of 0x0025.
18.6. New Error Response Codes
This usage does not define any new error response codes.
18.7. Client Procedures
Client procedures are defined in Section 7.1.
18.8. Server Procedures
Server procedures are defined in Section 7.2.
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Internet-Draft ICE March 200718.9. Security Considerations for Connectivity Check
Security considerations for the connectivity check are discussed in
Section 17.
19. IANA Considerations
This specification registers new SDP attributes and new STUN
attributes.
19.1. SDP Attributes
This specification defines seven new SDP attributes per the
procedures of Section 8.2.4 of [10]. The required information for
the registrations are included here.
19.1.1. candidate Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: candidate
Long Form: candidate
Type of Attribute: media level
Charset Considerations: The attribute is not subject to the charset
attribute.
Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and provides one of many possible candidate
addresses for communication. These addresses are validated with
an end-to-end connectivity check using Simple Traversal Underneath
NAT (STUN).
Appropriate Values: See Section 15 of RFC XXXX [Note to RFC-ed:
please replace XXXX with the RFC number of this specification].
19.1.2. remote-candidates Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: remote-candidates
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Long Form: remote-candidates
Type of Attribute: media level
Charset Considerations: The attribute is not subject to the charset
attribute.
Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and provides the identity of the remote
candidates that the offerer wishes the answerer to use in its
answer.
Appropriate Values: See Section 15 of RFC XXXX [Note to RFC-ed:
please replace XXXX with the RFC number of this specification].
19.1.3. ice-lite Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: ice-lite
Long Form: ice-lite
Type of Attribute: session level
Charset Considerations: The attribute is not subject to the charset
attribute.
Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and indicates that an agent has the minimum
functionality required to support ICE inter-operation with a peer
that has a full implementation.
Appropriate Values: See Section 15 of RFC XXXX [Note to RFC-ed:
please replace XXXX with the RFC number of this specification].
19.1.4. ice-mismatch Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: ice-mismatch
Long Form: ice-mismatch
Type of Attribute: session level
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Charset Considerations: The attribute is not subject to the charset
attribute.
Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and indicates that an agent is ICE capable,
but did not proceed with ICE due to a mismatch of candidates with
the default destination for media signaled in the SDP.
Appropriate Values: See Section 15 of RFC XXXX [Note to RFC-ed:
please replace XXXX with the RFC number of this specification].
19.1.5. ice-pwd Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: ice-pwd
Long Form: ice-pwd
Type of Attribute: session or media level
Charset Considerations: The attribute is not subject to the charset
attribute.
Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and provides the password used to protect
STUN connectivity checks.
Appropriate Values: See Section 15 of RFC XXXX [Note to RFC-ed:
please replace XXXX with the RFC number of this specification].
19.1.6. ice-ufrag Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: ice-ufrag
Long Form: ice-ufrag
Type of Attribute: session or media level
Charset Considerations: The attribute is not subject to the charset
attribute.
Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and provides the fragments used to construct
the username in STUN connectivity checks.
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Appropriate Values: See Section 15 of RFC XXXX [Note to RFC-ed:
please replace XXXX with the RFC number of this specification].
19.1.7. ice-options Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: ice-options
Long Form: ice-options
Type of Attribute: session level
Charset Considerations: The attribute is not subject to the charset
attribute.
Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and indicates the ICE options or extensions
used by the agent.
Appropriate Values: See Section 15 of RFC XXXX [Note to RFC-ed:
please replace XXXX with the RFC number of this specification].
19.2. STUN Attributes
This section registers two new STUN attributes per the procedures in
[12].
0x0024 PRIORITY
0x0025 USE-CANDIDATE
20. IAB Considerations
The IAB has studied the problem of "Unilateral Self Address Fixing",
which is the general process by which a agent attempts to determine
its address in another realm on the other side of a NAT through a
collaborative protocol reflection mechanism [21]. ICE is an example
of a protocol that performs this type of function. Interestingly,
the process for ICE is not unilateral, but bilateral, and the
difference has a signficant impact on the issues raised by IAB.
Indeed, ICE can be considered a B-SAF (Bilateral Self-Address Fixing)
protocol, rather than an UNSAF protocol. Regardless, the IAB has
mandated that any protocols developed for this purpose document a
specific set of considerations. This section meets those
requirements.
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Internet-Draft ICE March 200720.1. Problem Definition
From RFC 3424 any UNSAF proposal must provide:
Precise definition of a specific, limited-scope problem that is to
be solved with the UNSAF proposal. A short term fix should not be
generalized to solve other problems; this is why "short term fixes
usually aren't".
The specific problems being solved by ICE are:
Provide a means for two peers to determine the set of transport
addresses which can be used for communication.
Provide a means for resolving many of the limitations of other
UNSAF mechanisms by wrapping them in an additional layer of
processing (the ICE methodology).
Provide a means for a agent to determine an address that is
reachable by another peer with which it wishes to communicate.
20.2. Exit Strategy
From RFC 3424, any UNSAF proposal must provide:
Description of an exit strategy/transition plan. The better short
term fixes are the ones that will naturally see less and less use
as the appropriate technology is deployed.
ICE itself doesn't easily get phased out. However, it is useful even
in a globally connected Internet, to serve as a means for detecting
whether a router failure has temporarily disrupted connectivity, for
example. ICE also helps prevent certain security attacks which have
nothing to do with NAT. However, what ICE does is help phase out
other UNSAF mechanisms. ICE effectively selects amongst those
mechanisms, prioritizing ones that are better, and deprioritizing
ones that are worse. Local IPv6 addresses can be preferred. As NATs
begin to dissipate as IPv6 is introduced, server reflexive and
relayed candidates (both forms of UNSAF mechanisms) simply never get
used, because higher priority connectivity exists to the native host
candidates. Therefore, the servers get used less and less, and can
eventually be remove when their usage goes to zero.
Indeed, ICE can assist in the transition from IPv4 to IPv6. It can
be used to determine whether to use IPv6 or IPv4 when two dual-stack
hosts communicate with SIP (IPv6 gets used). It can also allow a
network with both 6to4 and native v6 connectivity to determine which
address to use when communicating with a peer.
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Internet-Draft ICE March 200720.3. Brittleness Introduced by ICE
From RFC3424, any UNSAF proposal must provide:
Discussion of specific issues that may render systems more
"brittle". For example, approaches that involve using data at
multiple network layers create more dependencies, increase
debugging challenges, and make it harder to transition.
ICE actually removes brittleness from existing UNSAF mechanisms. In
particular, traditional STUN (as described in RFC 3489 [15]) has
several points of brittleness. One of them is the discovery process
which requires a agent to try and classify the type of NAT it is
behind. This process is error-prone. With ICE, that discovery
process is simply not used. Rather than unilaterally assessing the
validity of the address, its validity is dynamically determined by
measuring connectivity to a peer. The process of determining
connectivity is very robust.
Another point of brittleness in traditional STUN and any other
unilateral mechanism is its absolute reliance on an additional
server. ICE makes use of a server for allocating unilateral
addresses, but allows agents to directly connect if possible.
Therefore, in some cases, the failure of a STUN server would still
allow for a call to progress when ICE is used.
Another point of brittleness in traditional STUN is that it assumes
that the STUN server is on the public Internet. Interestingly, with
ICE, that is not necessary. There can be a multitude of STUN servers
in a variety of address realms. ICE will discover the one that has
provided a usable address.
The most troubling point of brittleness in traditional STUN is that
it doesn't work in all network topologies. In cases where there is a
shared NAT between each agent and the STUN server, traditional STUN
may not work. With ICE, that restriction is removed.
Traditional STUN also introduces some security considerations.
Fortunately, those security considerations are also mitigated by ICE.
Consequently, ICE serves to repair the brittleness introduced in
other UNSAF mechanisms, and does not introduce any additional
brittleness into the system.
20.4. Requirements for a Long Term Solution
From RFC 3424, any UNSAF proposal must provide:
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Identify requirements for longer term, sound technical solutions
-- contribute to the process of finding the right longer term
solution.
Our conclusions from STUN remain unchanged. However, we feel ICE
actually helps because we believe it can be part of the long term
solution.
20.5. Issues with Existing NAPT Boxes
From RFC 3424, any UNSAF proposal must provide:
Discussion of the impact of the noted practical issues with
existing, deployed NA[P]Ts and experience reports.
A number of NAT boxes are now being deployed into the market which
try and provide "generic" ALG functionality. These generic ALGs hunt
for IP addresses, either in text or binary form within a packet, and
rewrite them if they match a binding. This interferes with
traditional STUN. However, the update to STUN [12] uses an encoding
which hides these binary addresses from generic ALGs.
Existing NAPT boxes have non-deterministic and typically short
expiration times for UDP-based bindings. This requires
implementations to send periodic keepalives to maintain those
bindings. ICE uses a default of 15s, which is a very conservative
estimate. Eventually, over time, as NAT boxes become compliant to
behave [34], this minimum keepalive will become deterministic and
well-known, and the ICE timers can be adjusted. Having a way to
discover and control the minimum keepalive interval would be far
better still.
21. Acknowledgements
The authors would like to thank Dan Wing, Eric Rescorla, Flemming
Andreasen, Rohan Mahy, Dean Willis, Eric Cooper, Jason Fischl,
Douglas Otis, Tim Moore, Jean-Francois Mule, and Francois Audet for
their comments and input. A special thanks goes to Bill May, who
suggested several of the concepts in this specification, Philip
Matthews, who suggested many of the key performance optimizations in
this specification, Eric Rescorla, who drafted the text in the
introduction, and Magnus Westerlund, for doing several detailed
reviews on the various revisions of this specification.
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is a minimalist implementation that does little but respond to STUN
checks.
Because ICE requires both endpoints to support it in order to bring
benefits to either endpoint, incremental deployment of ICE in a
network is more complicated. Many sessions involve an endpoint which
is, by itself, not behind a NAT and not one that would worry about
NAT traversal. A very common case is to have one endpoint that
requires NAT traversal (such as a VoIP hard phone or soft phone) make
a call to one of these devices. Even if the phone supports a full
ICE implementation, ICE won't be used at all if the other device
doesn't support it. The lite implementation allows for a low-cost
entry point for these devices. Once they support the lite
implementation, full implementations can connect to them and get the
full benefits of ICE.
Consequently, a lite implementation is only appropriate for devices
that will *always* be connected to the public Internet and have a
public IP address at which it can receive packets from any
correspondent. ICE will not function when a lite implementation is
placed behind a NAT.
It is important to note that the lite implementation was added to
this specification to provide a stepping stone to full
implementation. Even for devices that are always connected to the
public Internet, a full implementation is preferable if achievable.
A full implementation will reduce call setup times. Full
implementations also obtain the security benefits of ICE unrelated to
NAT traversal; in particular, the voice hammer attack described in
Section 17 is prevented only for full implementations, not lite.
Finally, it is often the case that a device which finds itself with a
public address today will be placed in a network tomorrow where it
will be behind a NAT. It is difficult to definitively know, over the
lifetime of a device or product, that it will always be used on the
public Internet. Full implementation provides assurance that
communications will always work.
Appendix B. Design Motivations
ICE contains a number of normative behaviors which may themselves be
simple, but derive from complicated or non-obvious thinking or use
cases which merit further discussion. Since these design motivations
are not neccesary to understand for purposes of implementation, they
are discussed here in an appendix to the specification. This section
is non-normative.
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STUN transactions used to gather candidates and to verify
connectivity are paced out at an approximate rate of one new
transaction every Ta milliseconds, where Ta has a default of 20ms.
Why are these transactions paced, and why was 20ms chosen as default?
Sending of these STUN requests will often have the effect of creating
bindings on NAT devices between the client and the STUN servers.
Experience has shown that many NAT devices have upper limits on the
rate at which they will create new bindings. Furthermore,
transmission of these packets on the network makes use of bandwidth
and needs to be rate limited by the agent. As a consequence, the
pacing ensures that the NAT devices does not get overloaded and that
traffic is kept at a reasonable rate.
B.2. Candidates with Multiple BasesSection 4.1.1 talks about merging together candidates that are
identical but have different bases. When can an agent have two
candidates that have the same IP address and port, but different
bases? Consider the topology of Figure 22:
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(10.0.1.1:2498) and a host candidate on its interface on network A
(192.168.1.1:3344). It performs a STUN query to its configured STUN
server from 192.168.1.1:3344. This query passes through the NAT,
which happens to assign the binding 10.0.1.1:2498. The STUN server
reflects this in the STUN Binding Response. Now, the offerer has
obtained a server reflexive candidate with a transport address that
is identical to a host candidate (10.0.1.1:2498). However, the
server reflexive candidate has a base of 192.168.1.1:3344, and the
host candidate has a base of 10.0.1.1:2498.
B.3. Purpose of the <rel-addr> and <rel-port> Attributes
The candidate attribute contains two values that are not used at all
by ICE itself - <rel-addr> and <rel-port>. Why is it present?
There are two motivations for its inclusion. The first is
diagnostic. It is very useful to know the relationship between the
different types of candidates. By including it, an agent can know
which relayed candidate is associated with which reflexive candidate,
which in turn is associated with a specific host candidate. When
checks for one candidate succeed and not the others, this provides
useful diagnostics on what is going on in the network.
The second reason has to do with off-path Quality of Service (QoS)
mechanisms. When ICE is used in environments such as PacketCable
2.0, proxies will, in addition to performing normal SIP operations,
inspect the SDP in SIP messages, and extract the IP address and port
for media traffic. They can then interact, through policy servers,
with access routers in the network, to establish guaranteed QoS for
the media flows. This QoS is provided by classifying the RTP traffic
based on 5-tuple, and then providing it a guaranteed rate, or marking
its Diffserv codepoints appropriately. When a residential NAT is
present, and a relayed candidate gets selected for media, this
relayed candidate will be a transport address on an actual STUN
relay. That address says nothing about the actual transport address
in the access router that would be used to classify packets for QoS
treatment. Rather, the server reflexive candidate towards that relay
is needed. By carrying the translation in the SDP, the proxy can use
that transport address to request QoS from the access router.
B.4. Importance of the STUN Username
ICE requires the usage of message integrity with STUN using its short
term credential functionality. The actual short term credential is
formed by exchanging username fragments in the SDP offer/answer
exchange. The need for this mechanism goes beyond just security; it
is actual required for correct operation of ICE in the first place.
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Consider agents L, R, and Z. L and R are within private enterprise 1,
which is using 10.0.0.0/8. Z is within private enterprise 2, which
is also using 10.0.0.0/8. As it turns out, R and Z both have IP
address 10.0.1.1. L sends an offer to Z. Z, in its answer, provides
L with its host candidates. In this case, those candidates are
10.0.1.1:8866 and 10.0.1.1:8877. As it turns out, R is in a session
at that same time, and is also using 10.0.1.1:8866 and 10.0.1.1:8877
as host candidates. This means that R is prepared to accept STUN
messages on those ports, just as Z is. L will send a STUN request to
10.0.1.1:8866 and and another to 10.0.1.1:8877. However, these do
not go to Z as expected. Instead, they go to R! If R just replied
to them, L would believe it has connectivity to Z, when in fact it
has connectivity to a completely different user, R. To fix this, the
STUN short term credential mechanisms are used. The username
fragments are sufficiently random that it is highly unlikely that R
would be using the same values as Z. Consequently, R would reject the
STUN request since the credentials were invalid. In essence, the
STUN username fragments provide a form of transient host identifiers,
bound to a particular offer/answer session.
An unfortunate consequence of the non-uniqueness of IP addresses is
that, in the above example, R might not even be an ICE agent. It
could be any host, and the port to which the STUN packet is directed
could be any ephemeral port on that host. If there is an application
listening on this socket for packets, and it is not prepared to
handle malformed packets for whatever protocol is in use, the
operation of that application could be affected. Fortunately, since
the ports exchanged in SDP are ephemeral and usually drawn from the
dynamic or registered range, the odds are good that the port is not
used to run a server on host R, but rather is the agent side of some
protocol. This decreases the probability of hitting an allocated
port, due to the transient nature of port usage in this range.
However, the possibility of a problem does exist, and network
deployers should be prepared for it. Note that this is not a problem
specific to ICE; stray packets can arrive at a port at any time for
any type of protocol, especially ones on the public Internet. As
such, this requirement is just restating a general design guideline
for Internet applications - be prepared for unknown packets on any
port.
B.5. The Candidate Pair Sequence Number Formula
The sequence number for a candidate pair has an odd form. It is:
pair priority = 2^32*MIN(O-P,A-P) + 2*MAX(O-P,A-P) + (O-P>A-P?1:0)
Why is this? When the candidate pairs are sorted based on this
value, the resulting sorting has the MAX/MIN property. This means
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that the pairs are first sorted based on decreasing value of the
maximum of the two sequence numbers. For pairs that have the same
value of the maximum sequence number, the minimum sequence number is
used to sort amongst them. If the max and the min sequence numbers
are the same, the offerers priority is used as the tie breaker in the
last part of the expression. The factor of 2*32 is used since the
priority of a single candidate is always less than 2*32, resulting in
the pair priority being a "concatenation" of the two component
priorities. This creates the desired sorting property.
B.6. The remote-candidates attribute
The a=remote-candidates attribute exists to eliminate a race
condition between the updated offer and the response to the STUN
Binding Request that moved a candidate into the Valid list. This
race condition is shown in Figure 23. On receipt of message 4, agent
L adds a candidate pair to the valid list. If there was only a
single media stream with a single component, agent L could now send
an updated offer. However, the check from agent R has not yet
generated a response, and agent R receives the updated offer (message
7) before getting the response (message 10). Thus, it does not yet
know that this particular pair is valid. To eliminate this
condition, the actual candidates at R that were selected by the
offerer (the remote candidates) are included in the offer itself.
Note, however, that agent R will not send media until it has received
this STUN response.
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upon. ICE defines a simple periodic keepalive that operates
independently of media transmission. This makes its bandwidth
requirements highly predictable, and thus amenable to QoS
reservations.
B.8. Why Prefer Peer Reflexive Candidates?Section 4.1.2 describes procedures for computing the priority of
candidate based on its type and local preferences. That section
requires that the type preference for peer reflexive candidates
always be lower than server reflexive. Why is that? The reason has
to do with the security considerations in Section 17. It is much
easier for an attacker to cause an agent to use a false server
reflexive candidate than it is for an attacker to cause an agent to
use a false peer reflexive candidate. Consequently, attacks against
the STUN binding discovery usage are thwarted by ICE by preferring
the peer reflexive candidates.
B.9. Why Send an Updated Offer?Section 11.1 describes rules for sending media. Both agents can send
media once ICE checks complete, without waiting for an updated offer.
Indeed, the only purpose of the updated offer is to "correct" the SDP
so that the default destination for media matches where media is
being sent based on ICE procedures (which will be the highest
priority nominated candidate pair).
This begs the question - why is the updated offer/answer exchange
needed at all? Indeed, in a pure offer/answer environment, it would
not be. The offerer and answerer will agree on the candidates to use
through ICE, and then can begin using them. As far as the agents
themselves are concerned, the updated offer/answer provides no new
information. However, in practice, numerous components along the
signaling path look at the SDP information. These include entities
performing off-path QoS reservations, NAT traversal components such
as ALGs and Session Border Controllers (SBCs) and diagnostic tools
that passively monitor the network. For these tools to continue to
function without change, the core property of SDP - that the
existing, pre-ICE definitions of the addresses used for media - the m
and c lines and the rtcp attribute - must be retained. For this
reason, an updated offer must be sent.
B.10. Why are Binding Indications Used for Keepalives?
Media keepalives are described in Section 10. These keepalives make
use of STUN when both endpoints are ICE capable. However, rather
than using a Binding Request transaction (which generates a
response), the keepalives use an Indication. Why is that?
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The primary reason has to do with network QoS mechanisms. Once media
begins flowing, network elements will assume that the media stream
has a fairly regular structure, making use of periodic packets at
fixed intervals, with the possibility of jitter. If an agent is
sending media packets, and then receives a Binding Request, it would
need to generate a response packet along with its media packets.
This will increase the actual bandwidth requirements for the 5-tuple
carrying the media packets, and introduce jitter in the delivery of
those packets. Analysis has shown that this is a concern in certain
layer 2 access networks that use fairly tight packet schedulers for
media.
Additionally, using a Binding Indication allows integrity to be
disabled, allowing for better performance. This is useful for large
scale endpoints, such as PSTN gateways and SBCs.
Author's Address
Jonathan Rosenberg
Cisco
Edison, NJ
US
Phone: +1 973 952-5000
Email: jdrosen@cisco.com
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